Why do they keep you in the fountain for 15 seconds?

Without the barrier, positional advantages in early invades, scouting and other pre-laning tactics becomes a matter of whose computer is more powerful and who bought their items faster. Keeping players in the fountain means that everyone can leave at the same time rather than a couple guys speeding out and a couple guys still in base.

Probably for the sake of making sure everyone starts at almost the same time.

Unless you're AFK, you're going to run out of that gate at the 15 second mark, as is every other player. As opposed to the past, when some people would take 2 seconds to click the few items they needed and sprint, leaving 5-second gate runners behind.

It doesn't sound like much of a difference, but it can have a pretty significant effect competitively.

Can anyone answer this?

i'm assuming it's to give LCS players and players in general time to buy early items before letting you position yourself on the map, literally 2 seconds of positioning in LCS is incredibly important in the first 3 min of the game.

I think They added it so invades would not be as common for 5 man premades since they could get into enemy jungle before anyone else got there

It's like releasing caged animals to fight to the death. Remember back when gladiators would fight in the arena? They were held behind doors before they both opened and they proceeded to kill each other. Some concept.

Or I could be completely wrong

You ever seen a rammus teleport hide in bush gank in super early game?