It's the same reason successful tradespeople often hire someone to perform estimates. They're delegating low hanging tasks so that they can focus on money making activities. A VA allows you to take more jobs because you're spending less time chasing clients for scheduling, payment, fielding questions, etc.

The nice thing about the trades is it offers an opportunity to work for yourself and run your own business but for, again, I'll say, most tradespeople, they really could use some classes on how to run their business optimally.