Well, here are two reasons to use crossfades..
- If you make an edit and have two Regions joined, sometimes the waveform is interrupted at the edit point and jumps in level -- this can cause a small but noticeable click in the audio. Inserting a short [10 millisecond or so] crossfade at the join can eliminate the click [see screenshot below].
- If you want a gradual fade between two songs, a longer crossfade is one way to do it.