Because there is a thing called Min-Maxing in videogames, it's present in all videogames that have stats in them, you want to reach the cap of 9999 hp in FF III?
minmaxing
Do you want to get the most of a certain situation spending in the meantime the less resources possible?
minmaxing
if you play lux or god have mercy on your soul veigar support, well, annie which is an actual solo laner/support can do THE SAME EXACT THINGS just better, easier and with less costs
You don't have to take for gospel whatever comes out from the mouth of a certain pro player or god have mercy on their souls as well, shitty yourubers, but playing things like yasuo adc, teemo jungle is just sub optimal, you're reducing the numbers in your favor, playing meta, understanding the meta and MOST IMPORTANT OF ALL understanding the game is the key to win, now you have to understand what's meta and what's trolling, because if you play an unpopular jungler like let's say sejuani right now that could definitely use some buffs you're not trolling, you're just using a supposed sub optimal pick, but if you know what you're doing you can do as well as everybody else if not better.
Now let's say you're playing twitch/singed/whatever jungle, you might be lured by noobtraps thinking he's actually good in that role but he isn't, and the reason why it might work it's just because the enemy team is inexperienced enough to let you get away with it (consider that below diamond 3 people are most likely garbage so they don't count and sometimes even above it), this way you're putting your teammates in an uncomfortable position and they'll have to put more effort into winning just because you're not helping them towards your common goal.
runes are subjective and since they're just numbers you're adding to your characters is commonly recommended to follow what pro players do since they did the math before and you won't waste time reinventing the wheel, some runes are pretty standard as for an ADC since if you don't have the +5 damage mastery and the red AD runes, you won't be able to last hit minions if they get under your tower as they need 2 turret hits and 1 auto for the melees and 1 turret hit 1 auto from you or your support and then the last hit, and believe me, if i manage to create a lead of 12 minions (2 waves) you're done playing for that game as i will stomp you (hoping my teammates won't be dumb enough to screw me up and make me lose that lead)
Now, don't tell me this was hard to figure out, will you?
http://i.imgur.com/fAUOr2c.png
