As a new writer, it seems pretty obvious: you write 20 chapters of 4,000-5,000 words each and you have a book of 80,000-100,000 words. What could be be easier? What could go wrong?
1. A novel made up of consistently 4,000-5,000 words would be dull to read. You want to mix it up a bit, a few short chapters here, a few long chapters there. Keep the reader on their toes so they don't know what's coming next.
2. Chapters can control pace. Short chapters speed things up, long chapters slow them down. A long chapter at a relatively action free moment will cause the pace to falter. A series of super short chapters at a very exciting moment will make the pace so fast you risk losing the reader as they can't take it all in.
3. If you've planned your novel out in chapters, you've almost certainly made the end of each chapter the end of a scene ie the action has peaked and fallen. Chapter ends are what keep the reader reading as they think, hmm, I'll just start the next chapter and see what happens - and then they're hooked. Good chapter ends are often made by ruthlessly cutting scenes short so they end at a tense moment.
4. Having your chapters planned out from the start makes you more reluctant to re-write and move scenes around. All books need this to a certain extent and some - such as memoirs - often benefit from really moving scenes around so you go middle, beginning, end, rather than beginning, middle, end. You won't do this if it mucks up your beautiful chapter plan, even though the story demands it.
5. Chapters are a way of breaking a novel down into nicely manageable chunks for the reader - you yourself may read a chapter or two each evening before you go to sleep. But you're not the reader any more, you're the writer and you want the reader to stay up all night hooked on reading your fabulous book. Thinking of it in scenes will help in this.
If this seems scary, make each scene you write a chapter, and then later combine them. When you've sorted out the story you can play around with where the chapters should come for the most impact.