It allows you to plan out what happens during/around it. For example, should my leisure time the day before be something more fun but cognitively demanding or more chill and relaxing?

It also allows you to get in the right state of mind for the work. In my experience (is this also an autistic thing? I don’t know), if you’re mentally prepared for something very difficult and unpleasant, it greatly cuts down on how unpleasant it is, sometimes even turning that difficult thing into a fun challenge. If you mentally prepare for something that’s worse than what’s actually ahead, you end up with way too much excess energy and the need to look for problems to solve even when no problems exist.