Koa is developed by the team behind a widely used node.js framework (express.js).

Using generators (a bleeding edge feature, even for Node.js) would clean up your code from the mess caused by all those callbacks; making your code more manageable.

Koa is very lightweight with just 550 lines of code.

Generators functions are of course a huge plus, but at the time Koa team has transcended generations functions and shifted towards async/await style programming. It has made the Koa best framework available in the market.