Well, part of it is because we're trying to promote our Costume-Con, but that's only a small part of it. I would do room parties even if we weren't running a con in a few years.
For me, room parties, and particularly open room parties are an important part of convention culture.
My introduction to fandom was through the SCA, a very do-it-yourself group. There were events put on by volunteers, and after the events were usually "post-revels;" parties with relatively open invite lists that happened at participants' houses. It was just part of the process of winding down after a long day.
My first convention was Minicon 28, and back in those days, Minicon was a party convention. Nearly every suite in the hotel and nearly every room surrounding the pool hosted an open room party (and it was in a really big hotel; there were literally dozens each night). I went to Minicon for 6 years, and then it changed so I stopped going. But while I went, I went with groups of people who hosted parties and sometimes helped. Most of the other conventions I went to at the time were also big party scenes, and I met other people who ran parties.
At WorldCon, I discovered "The Costumers' Suite," a privately run open hospitality suite. At NASFiC in Los Angeles, the gal running it got roped in at the last minute, so we looked at things and said "In San José, we can do better." We put together a schedule of events, with groups hosting 4 nights and 2 afternoons, and a private-ish party Monday night at the end. We learned a lot about decorating and transforming a room from some of the people who came in; both the Klingons and the GBACG Faeries did a beautiful job making the room look like something other than a hotel suite parlor.
That was also about the time that we and the A.C.R.O.N.Y.M. gals decided to do a BayCon party together, this quickly split into the "Evil Geniuses" party on Friday night and the "Den of Propriety" party on Saturday. The "Evil Geniuses" party quickly became a great format to get visibility and help us promote our Costume-Con bid.
So it all comes back to fandom being a do-it-yourself group. I don't really want to be a con staffer all the time (even though I do take a staff job once in a while). I can be a good panelist and moderator. I can throw a good party. I think those are important contributions back to con-going fandom.