Heat will speed up the processing/activation of bleach. This is why “hot roots” happen; the hair closest to your scalp processes/lifts faster because the hair next to your head is the warmest due to the heat of your scalp. So, heat will help speed up the process, and may help prevent hot roots if you’re using heat on the lengths to help maintain an even temperature overall, but is not necessary.
If you’re starting from a lighter base color you may not want to use heat processing to speed things up since you will have less control over the lifting time!