I'm kinda' a noob... so I might just not understand... but why is the META the way it is...? Why is the META tank top, fighter/mage mid, marksman and support bot, and a jungler...? What exactly makes this so effective and specific...? :P
Having 1 jungler is for gold/xp efficiency; nobody in the jungle means xp not going to anyone, and lane xp being shared. The Support starves himself so he's not fighting his carry for CS (fighting over it only results in lost CS and wasted abilities), and the typical Support can operate without having lots of gold. This meta didn't always exist, but having a meta established is good for strangers playing together; it reduces confusion of what each person's job is without having an argument about it, and it works. If your opponent does something off-meta, they often do it at a disadvantage; if they gain an advantage from it, the enemy usually just compensates by copying it (like sending the support to top) making it pointlessly even again.
In the end, it works, and is usually the best way to have order among a group of complete strangers who've never met. If there's no meta it'd be complete chaos (not that there isn't ever chaos now, but it helps).
Short answer: The pros find it to be most effective.
Long answer: Basically the idea of having a tank top was because they're further away from most of the fights that happen and are hard to kill. They beef up for team fights (tp for drag etc) mage mid was huge in s4 because blue buff was really strong on them, and you generally want an AP champ doing damage so they cant stack armor vs your AD's. Marksman and support bot is popular because the support protects the ADC and helps him get efficient farm to do a lot of damage mid-late game when the consistent damage is needed, Generally ADC's want 2-3 items before they start grouping. In the pro scene the jungler is USUALLY a 2nd support. He is unseen most of the time and can place wards in spots the support cant. (due to this the wards are generally harder to find because thers no clear "mia") Like a support, the jungler works to get his teamates ahead and most importantly secure objectives whether it be turrets, dragon, baron, inhibs etc.
Okay so there are two parts to the meta:
1-1-2-J
and
Tank<DPS<Burst<Tank
The first part is to maximize gold efficiency. Naturally, there being 3 lanes with 3 individual streams of minions (and therefore exp+gold), you want to get at least one laner on each lane. If you have a Jungler, you are now taking advantage of a 4th stream of gold, the Jungle. This unfortunately leaves one champion starved of at least gold, so we tend to have one last champion who's really good at supporting the team even when on a lack of budget (which, somewhat obviously, is our Support). So, we have 1 guy top, 1 guy mid, 1 ADC bot, 1 support to protect the ADC bot, and a Jungler. As to why the duo lane goes bot, notice the Shyvana looking creature near bot lane.
The second part is role diversity. In some way, you always want at least one DPS carry on your team (usually the ADC), at least one Burst carry on your team (usually the mid laner), and at least one team fight orientated tank (Top, Jungle, and/or Support). DPS carries scale best on gold and don't care as much about levels, so it's perfectly safe for them to lane with someone leeching their EXP so long as they get the gold. This is where we get the traditional ADC+Support Bot lane. Burst Carries tend to scale really well on levels so a solo lane is preferable, but they are just as prone to death as an ADC, so we tend to put them in mid lane as it provides them the most protection (mid lane is the shortest lane, and therefore easiest to reach tower with). Tanks, finally, tend to be much more flexible and diverse than their damage carry counterparts, and are usually well adapted to defending themselves for extended periods of time (Top), constantly fighting and setting up ganks (Jungle), or operating with a limited gold budget (support). A balanced team will almost always want at least one member in every asset of this balance triangle, as it will prepare them for whatever the enemy team throws at them if played correctly.
Riot doesn't want assassins in the game. They don't like people getting bursted, because the people who get bursted complain they didn't have any window of opportunity to escape death.
I see both sides.
The assassins.. are the opportunistic hunters who like to find solitary 1v1 duels within a given game (or 2v1 if they can burst the first target down fast enough). The ones who get bursted feel this isn't fair, but assassins are made for one thing; to kill things. What does every role besides assassins do? Provide team-based opportunism. Assassins provide solitary opportunity. This game isn't about solitary engagement, it's about big team fights under someone's tower or another objective. It's a complete oxymoron to even have assassins in the game at this point.
Riot cant balance assassins in a team-based game so they turn the meta into forced team-fights, tank-initiated team skirmishes... i personally hope this game goes in the crapper where it belongs. Been playing for 4 years now and all i see is imbalance after imbalance when they call it 'balance.' Cinderhulk is a prime example. Look at Shyvanna's win/play rate comparison before and after the cinderhulk buff. So from ONE item getting buff a champion automatically becomes TOP tier play/win rate again? How is that balancing the game? All Riot does with each and every patch is kick the can down the road..
The reason to have a Jungler is to gain the XP and Gold from Jungle creeps, and increase the Over all value of your team.
The reason ADC's are paired with the supports, is that They don't rely so much on levels as much as Items. They can have have a second leaching XP as long as they get the gold. Otherwise, they could do alright mid. Mid lane works well for squishies like the mage/assassin/ADC because it's the shortest lane. giving better protection from your towers. Mages just became associated with mid mostly because of Blue buff.
Having double top or trilane somewhere is also a valid strategy (you can also see double jungle in LCS games sometimes), if you pick good early game champions that can escape ganks/dives and can push towers. But these strategies are not very fun for the opposing team which will follow the meta.
