I can remember the days of 4 track recording and bouncing tracks. With something like Sonar with pretty much unlimited tracks what would be the purpose of bouncing and when should it be done?

Before SONAR could freeze tracks, we used to bounce tracks and archive the original tracks to free up memory and CPU. Now freeze handles that task. Bouncing and freezing still let you create a track that is a snapshot in time. That can be useful for synths and effects that generate "random" results.

It's two separate things really.  On 4-track tape you would bounce and mix three tracks into one in order to free up tracks for re-use.  In Sonar, you bounce or freeze midi+softsynth to audio or audio+effects to processed audio for reasons mentioned by Scook above.

You bounce when you want to combine 1 or more tracks together or combine 2 or more clips together.
You can bounce if you want to print an effect on a track. Or to save CPU. That's why its there.....

Can you imagine if there wasn't!!  Everyone would complain on why there is no bounce setting in Sonar. Every single program has a bounce in it. Its part of recording and mixing.

Why would sonar leave it out, just because you have unlimited track counts? There is a plethora of different scenarios that you can use bounce.