Many authors rely on school visits to make up as a substantial part of their income. Indeed, many authors earn more from running events and workshops than they do from writing, and could not afford to write full time without this income.
There are many other benefits to authors beyond the financial though. Here are just a few of them.
Direct contact with your audience. There are few groups more honest than a class full of school children. Want to know if your last book was any good? They'll tell you. If they liked it they'll rave about it, and if they didn't you'll have a list of things not to do in your next book!
Brainstorming. Sometimes just standing up and talking about books and writing can help you come up with new ones. Tell your audience about your next book and you'll know immediately if they're interested or not. Once you've started explaining the story, you can't just stop, so often you're forced to take the plot in new directions you hadn't dreamed of in order to get them interested again.
Inspiring future generations of writers. OK, it's a bit cheesy, but it's true. Every classroom you visit may contain a future novelist, illustrator or screenwriter. A visit from you can help inspire them in ways few other people ever could. You could be the one to set them on their creative path.
Of course, there's no escaping the financial incentives, too. One or two events a week could be enough to allow a part-time writer to go full-time, which means more writing time, and more books on the shelves.