Well there are different reasons for pruning, and different types of pruning. Removing large hazardous or dead limbs, corrective pruning for trees that have been hacked in the past, pruning to develop good structure, thinning and endweight reduction to keep trees from growing out of controll and blowing over or breaking in wind and storms, asthetic pruning to keep trees looking good, and more, plus any combination of these.
Heres a tree that I did, it was hacked(all main branches chopped of to stubs in the past) I cleaned out all dead wood, reduced endweight on heavy branches, thinned the canopy to allow the wind to pass through without blowing the tree over or breaking branches, removed crossing or competing leaders and branches etc.... complete prune really, the customer lost one just like it to the wind last year so he wanted to do something to help this one. I actually talked him out of topping it agian and he ended up really happy with the look and said it did great in the 60 mph+ winds we had recently.
before:
http://i8.photobucket.com/albums/a23...c/DSC00241.jpg 
After
http://i8.photobucket.com/albums/a23...c/DSC00244.jpg 