As you can see from the last couple of posts by Jim and Daveeb Simon, a router is independant of your PC.
So once its set up if you format or change your pc then you dont have to reinstall the modem, just plug the ethernet cable in and youre online.
This makes them much preferable to have even if you arent thinking of networking.
If you do decide at a later date to get a laptop or a second pc to network then theyre fantastic.
The connection is usually much more stable and often faster as they use ethernet not USB, without USBs inherent problems and system resources usage.
As you occassionally build and repair pcs it also means that you can have your pc on line while you search for drivers etc and also have the one that you are building or working on online.
You will pobably find an ADSL 2+ moden/router, with or without wi fi for about the same price as you would pay for a decent USB modem.
No contest really even if it costs you a tenner or so more