Leashing isn't about the jungler 'needing' that leash in order to clear.
It's about safety vs cost.
It costs you very little to leash. If done correctly, you'll lose little health. Your 1st cs will heal that back if you have Feast mastery, or enough regen in your support stuff if you're a tank support and you tank the 1st couple hits from it.
And you should still be in lane in time for 1st minion to die, which you can hit at the very last second and get the cs from it, and support should be in range by then to get exp from it too.
So what exactly has it cost you to leash?
You gain a huge amount of safety, not only for your jungler, but for your team as a whole.
You're making it dangerous for the enemy to invade by being there.
If the enemy invades, and you're not there to counter it, then your lanes are going to be vulnerable too, because they'll be able to gank you from places your team can't ward yet.
So, the real question is, what reason do you have to not leash?
Are you bad at last hitting?
Are you going to fail to get that 1st cs because you don't know how to path correctly/know the correct time to leave leash and head for lane?
What can you gain from being in lane before minions are in lane?
Jungle camps spawn at the exact point your minions reach outer tower, and you have several seconds before they actually start killing each other.
-So, what exactly are you going to do in that time?
If it's not leashing/watching jungle for invades, then you are just wasting time being a costly ward for your tower, which in itself is already a ward.
Might as well double ward that inner brush by inhib tower too while you're at it.
It's just a waste of team resources to do anything else other than leash/watch jungle.
This isn't season 1 where everyone just ran to stand next to their respective outer towers to wait 3minutes for the game to actually start.
The game starts as as soon as spawn pool gate opens. Go do something useful.