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Well, Logic does allow many variations of workflow...
For example if you had two audio regions on one track. Now you want to try out what region 2 sounds like before region 1 ends. If you had "No Overlap mode" on then dragging the 2nd region over the first would shorten the first.
Useful if you want to create a crossfade at that point without the extra steps of shortening the edges of the regions to fit against each other.
Often I have MIDI regions for drum parts. Say I want to combine a Kick region with a hi hat region without creating extra tracks in the Arrange... using "Overlap" mode you can drag one MIDI region ontop of the other and both all notes in both regions will be triggered.
These are just two examples. I normally stick to Overlap mode with audio regions. But remember only the audio region on top is played by that particular channel.
Hope that helps,