They add dll files needed by the runtime to run. Without them you'd get an error.
That said, not all the dll in the redistributes are needed. You can look at your game exe with something like "dependency walker" to see all the dlls it uses. Notably the dx vs dlls are the one's you could bundle. Unless of course those dlls need some other dll.
Try this:
Put your game on a machine where you haven't installed those redists (hopefully nothing else has already or you'll miss stuff). Next, try and run the game and it will complain about a missing dll. Find that dll on another machine you have already installed the redists on, and copy it over to the same folder as your game. Then try running the game again and repeat till the game runs. All the dlls you copied over should be all you need to bundle with your game to run without the redists.