In the (good?) old days when we recorded on 4 or 8-track tape, bouncing tracks was a (not necessarily) evil necessity. As it forced you to commit to part of the mix early on, knowing there’d be no “undo” once you started to record new parts, it made you work that little bit harder to make sure the bounce was right, as well as making you protective of those tracks and wanting to ensure that you got the biggest impact with what was on them.
Of course, nowadays, recording in the box means we have an infinite number of tracks, but there are times when you might want to bounce down, simply to save resources on a mix that’s got out of hand, to “print” a track or tracks with all of the edits and plugins on it if your system is struggling to keep up. Another reason is to combine multiple mono sources into a single stereo file that you can then treat as a single source. Of course, you could direct all those mono sources to a stereo bus or group channel and add your EQ and effects to that instead, but sometimes it’s just easier to see a stereo track rather than all those mono sources in your project window. This video shows how to bounce a couple of mono drum overheads together to create a single stereo file, prior to mixing.