I have several reasons for working this way:
-I prefer to jump from one task to another in real life as well as in writing.
-My first two novels were mentorship projects with deadlines, and I had to make sure those revisions got done.
-Changes I make early in the manuscript are likely to affect what I write later.
-Visiting earlier chapters while I work on later ones keeps me aware of what happens everywhere in the manuscript and helps me keep the voice and characterization consistent.
But the main reason is that coming up with the original story is fun and I want to make it last. There will be plenty more revisions to make after I type the triumphant last line, and I'll enjoy them in their own way, but this method cuts down on the number of days in a row I'll end up reading the same chapters.
How about you? Whether you write novels, essays, emails, or anything else, when does revision happen?