I understand why does it exist in current game, but I don't really understand the reasoning behind the game requiring it to exist. I consider that the early game would benefit from the leash not being a thing anymore, as you would need to do some scouting to know where the enemy jungler will start. Now, things are pretty bland. You see bot lane reaching their lane later, you expect a top gank at around 3 minutes into the game. While, it can create room for some mind games, like starting without a leash, but have the opposite laner go in later, so it would fake a leash, I think that simply not knowing it's better.
So, back to my question. Why is it a thing? I feel that in Season 7, the timers changes kinda moved towards eliminating it, but it did not disappear. In SoloQ, at least, it can be a bit annoying sometimes. I often see lanes lost due to people not knowing how to leash, they end up staying too much, and they lose first 3 melee minions. And that might not seem like a lot, but if they don't take into consideration champions with good lvl 2 all-ins like Leona, Riven, they can get screwed up pretty easily.
If you ask me, I think that leashing should not be a thing anymore, and game should move towards eliminating the possibility of leashing, or the effectiveness of it. I would personally modify the Smite, so it returns flat amount of HP, so champions that are usually not so healthy in the jungle would benefit more from it than junglers that are having pretty healthy clears anyway, or a buff on starting items for junglers, so they won't struggle so much if they don't get a leash.