Tristana can ult minions
Why?
Why?
to show that minion who has the biggest rocket in their pocket.
Helps for clearing super-minions off nexus or nexus turrets in late stage desperate situations, when you can't kill them fast enough to stop the destruction of property otherwise.
I agree, make it so Tristana can't ult minions.
And make it so Cho can't ult minions either, that needs to be fixed too.
'cuz, they're the same size as she is
So you don't miss the cannon minion in lane.
The pick up range of the target she R's has use for this. You could ult a minion to catch multiple people, or head off someone, depending.
It's ridiculously niche to the point of being removable, but there are ways to use it.
I know that vayne can condemn minions to jungle. But this is pure wierd. Was there any time this was useful?
If you place a bomb on a minion and use your ult to set it off the radius is doubled. Would be more useful if it is exploded on a champ, but sometimes it's situational. 
I put my bomb on a minion and ulted it to kill that one bastard Caitlyn who was just out of my range.
If you're being sieged you can ult the minions away so the enemy champions draw turret aggro.
Riot thought people would pull off #LCSBIGPLAYS by ulting minions with bombs on them into the enemy. I'm not joking, this is actually why that is a thing.
You won't be saying "why" when you're on 10 health and that minion is gunning for your ass ;)
It's useful for those special ADCs who have difficulty last hitting the cannon minions :O
Soraka can heal minions. Stuff is weird.
because i am not missing that cannon minion