There are 7.1 receivers because having 7 speakers can create a more enveloping sound field with speakers at your front, sides, and behind you. There are currently no 7.1 discrete recordings but that doesn't mean you can't get 7.1 sound.
Receivers have many options for expanding 2 channel or 5.1 channel mixes to 6.1 or 7.1. 'All channel stereo' modes take 2 channel mixes and expand them to 5, 6 or 7 channels. Dolby Prologic II expands 2 channel to 5.1. DPLIIx expands 2 channel analog or digital to 6.1 or 7.1 and can also expand 5.1 to either 6.1 or 7.1. DPLIIx can be layered on top of DTS 5.1 or 6.1 to create 7.1, etc...
So the number of discrete channels recorded on a disc has only a limited relationship to how many channels you can get for playback. There are no discs in any format that contain 7.1 discrete channels, but that may change in the future so having a 7.1 receiver is useful to be ready for future 7.1 formats should they ever arrive. A 7.1 receiver is also useful now if you want to use the processing modes described above to get 7.1 sound from 2,5.1, or 6.1 sources. Additionally, many 7.1 receivers can be configured to play 5.1 in the main room and use the other two amps to drive stereo speakers in another room. Lots of good reasons to choose a 7.1 receiver in my opinion.