Its a copy protection to try to keep piracy down, and also to distribute the game faster to the masses that dont want to go to the store to pick it up and want it available to dnload on their machines.
Great idea but steam with some games causes more issues than not.

I purchased FO NV via steam so I could play it on release night when I realized I didnt pay for overnight shipping of FO NV's collectors edition plus I wanted a hard copy just in case the event might happen that the game and steam didnt play nice.... Well of course steam causes issues you have to disable everything and some even have to play FO NV in offline mode. Well Little did I know that FO NV my hard copy had steam incorperated into it... not a happy camper at this point but what are you going to do.... got the physical copy just incase the game was the pita with steam..

As for the copy protection and all if you put out a good game that is not fully bug free but for the most part bug free. People are going to buy it, you put out a good game that most pc users that dont know how to do work arounds/ make things work are not going to pay the 50 bucks plus for a game and will dnload it off some site without paying a dime. Then you have people that just dont want to pay for anything that will do that anyways.... Me I know how to get around the bugs that many poeple are having even the crash instances in the Ultra Lux in the bath house.... Each play though though I do hate doing that quest in there.... because I know I might crash at least once as I am making my daring exit to avoid the crash not looking at the pool strifing right until i can get to the exit door with my face/camera in first person against the wall.... have the thorn all figured out too=/ But if companys put out good games/fewer major bugs more people would buy them and copy protection would not be needed. Good games get support by the consumer base. I am not calling FO NV a bad game I love it, loved FO3, but like many people on the EVGA Forums and being in the game industry myself as an environment artist/modeler I do know how to get squeeze by issues that come up with things like the Ultra Lux's bath house, that the average pc user normally runs into and cannot figure out. Even installing/uninstalling drivers the correct way link is in my sig, makes for alot less problems for gaming. Also knowing some audio drivers say for realtek esp the newer ones will cause a whole slew of issues with games have had that one come up time and time again on the evga forums now for the past 6-8 months, have to get people to unisntall them and go back to an older driver.... But yeah steam is a faster distribution/copy protection for FO NV that adds some un-needed issues for the game esp those who got hard copys.