PTJ gave part of the answer, although many modern true recording consoles are "in-line" style, so that the tape returns actually come to a second smaller fader on the same channel strip.
But there are other possible reasons why you might need more mixer channels than recording tracks:
1) You may want to use multiple mics on a source and then submix them (using a submix buss) to only one or two tape tracks.
2) You may be using a sequencer which is playing various midi devices. To save on precious tape tracks, you don't actually record the midi generated audio to the multitrack, but keep them as virtual tracks until the final stereo mixdown. But you still need to run them through the board so they can be heard and mixed. Depending on how many synth or drum machine parts you have, this could use up many additional mixer channels.
3) You may have other devices you might want to route through extra board channels: CD players, talkback mics, cassette decks, drum machine generated clicks, etc.
4) Sometimes it is useful to route the audio from a particular track to two different mixer channels (called "multing"), so that each can be processed or effected differently.
5) You might someday increase your track count.
So you can see there are many ways in which "extra" mixer channels can be useful. How many extra depends on your particular recording needs andn recording style.
