When is it necessary to enable "Time Remap" in After Effects?
If you are using Time Remap, a Timewarp should not be needed and vice versa.  Timewarp is designed to allow adjusting the playback rate, where as Time Remap is designed to allow you to tell the playback to be at a particular point in the original timecode when it reaches a particular time in your composition.  It mostly just matters how you want to control the adjustments and what level of control you want over the processing, but only one is needed.