print.
Say I had a model and one part needed to be longer. At the moment I can only change the size of the whole model. But if I could select just the part of the model that needed resizing it would save a lot of time and effort and I wouldn't have to remodel it in a cad package.
Likewise if i wanted to see if a specific part of a model would print - I could select that and not have to print the rest of the model around it. Which would save a lot of time and plastic.
It would just be a super useful thing to be able to do.