As Jason mentioned know Maxim will do the same thing, as will most other pieces of imaging software. However, I use Registar, as it never fails to align images (I've had some stacks where Maxim failed to align). I've also used it to find things in my images (like the soap bubble nebua). It will also do more than just overlap and align images, it will microscopically morph each stack so that the stars match exactly. It also is great for Mosaics. A 1 x 2 or 2 x 1 mosaic is pretty easy to assemble, but a 2 x 2 is much more difficult. I used it for a 2 x 4 mosaic and it matched the overlap perfectly, every time.
Is it needed, probably not. Is it worth the money, Absolutely!