Junglers being leashed has been a strategy in League of Legends for over a decade. The idea is that it's to help the jungler clear faster, take limited health deficit, and get to their other camps quicker so that they're in a position to gank. In other cases, like with Karthus, for example, you simply need the leash as the champion is not able to clear the starting buff at all. 

But now, players are starting to call for leashes to be completely abolished as season 15 rolls on. Players have taken to Reddit to express their annoyance at still having to leash for their jungler at level two as opposed to being able to get to their lane on time. The original poster said "If your Jungler has smite, and a Jungler Starting Item, they do NOT need leash. Just cover the jungle entrances, and play for level 2 advantag." And ultimately, he is right, because almost every jungler in the game does not need a leash, and if its the bottom lane having to leash, they're putting themselves at a disadvantage. 

Having the level two advantage in the bottom lane can be crucial to the lane actually being a winning one for you and your support. The original poster went on to say "By doing a leash as an adc/supp, you will arrive later to the lane than your opponent, throwing away a possible advantage to level up faster than them, and be unable to bully them outside lane."

One of the reasons players still leash is due to the fact junglers can, and have, intentionally thrown away games just because their bottom or top lane has not helped them to leash. They will then proceed to either decide they're not ganking your lane, or, in the worst case scenario, troll the game just because you didn't help them after they asked. This is a real problem, especially for players in the lower elo.