When is it necessary to enable "Time Remap" in After Effects? In After Effects to speed up and slow down footage this tutorial shows that you first enable Time Remap then use Timewarp. He doesn't appear to do anything with the Time Remap though.

This other tutorial shows doing slow motion either with the Stretch or with Timewarp

I get the Stretch vs Timewarp a bit but don't understand the Time Remap. When, if ever, should I be toggling that? From this nice written tutorial on time remapping it seems that Time Remapping is more useful if you need to reverse playback or add still frames and such. So was there a reason the Time Remap was toggled before the first guy did Timewarp? Does it make it render faster or work better or was it completely unnecessary?
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.