Your recordings and mixes will be affected by the room you're in. If the room magnifies certain frequencies, you'll naturally tend to cut those frequencies out of your mix (after all, your mixes sound like they have too much, right?). When you listen to your mix in another room, however, you'll realize that you cut where you shouldn't have. Getting your room acoustics under control (so you hear a balance between low, mid, and high frequencies) is important for these reasons.
Virtual MR1 is a handy, free tone generator that will help you test for inconsistencies in your room acoustics.
Generally, mid and high frequencies are easier to control (absorb) than lows. Carpet and drapes don't absorb enough lows to really help. In the meantime, they suck up tons of highs. The result is an unbalanced room... not a good thing. If you find that your best mix doesn't sound so good in the car, or on the home stereo or boombox, you may be mixing in an unbalanced room.
To get an idea of how you can acoustically treat your own studio, read how Willy did it himself in Willy's Studio Thread.