Same reason most infrastructure as a service companies exist.

Setting up, maintaining, and scaling web socket messaging servers isn't my core business. Accordingly I am happy to pay a reasonable fee to somebody like Pusher to have the problem go away.

If real-time messaging is core/fundamental to your business, then you probably should run your own servers. Otherwise let somebody else deal with that bit of complexity and focus on the parts of your business that matter most.

It's cheap, it's already working in my project, it has a nice test stub, automatic reconnections, flash fallback for older browsers, support is responsive and effective, even when it's my fault.

The many toy pusher clones need to worry about the 90% of a project that is the polish before pusher will go away.

If you can make it by yourself and for free, why not? This uses socket.io, it just makes it easier for you to mount your own push/pull service.