There is nothing to do with them, they have no purpose. The only reward for doing Achievements is that some give you a title or a Dye Station colour, that's it.

As in WOW, their main use is by the elitists who consider you a 'scrub' if you have less than an arbitrary number, often 10,000 but sometimes as high as 15,000 or even more.

It's an e-peen thing, nothing more.