Lets say that I want to take an 18 TU action, and I only have 16 TU's left. What I'd want to do is to start it now, and know that next turn, I will be 2 TU's down, and have it finished.
Right now, because of the arbitrary chunking nature of a turn, I can't, I have to waste 16 TU's now, and next turn I'm 18 TU's down when it finishes.
Why not permit someone to start something now that won't finish until next turn?
(In my case, it was reloading a weapon.)