I think you mean that those buttons are on the right side of the mixer, just before the main mix output faders. The big clue is that underneath them is the word SUBGROUPS. Subgroups are a useful intermediate step between the channel faders and the main mix.
The signal flow is as follows. Each channel fader can be assigned to submix 1-2, submix 3-4, or the main outputs (with the 1-2, 3-4, and MAIN buttons respectively, as you noted). These are known as busses. Each bus contains the sum of all channels assigned to it. This is useful in many cases. For instance, you might assign six channels of drums to submix 1-2, so you can control their overall volume together, with the submix fader. Then you might have four more channels, this time of guitar, assigned to submix 3-4. And so on.
Normally you would have both buttons above the subgroups fader depressed, so that the sound in that sub bus is sent in stereo to the main outs. But there are other cases where you might not want this sound in the main mix, for example when setting up some complex auxiliary routing. It's nice to have the option of taking the sound of that sub out of the main mix. But in any case it is still available on the back panel; at the SUB OUTPUTS jacks.
The confusing part is that each sub channel has two identical outputs on the back. The output of channel 1 can also be on channel 5. I don't see the point, myself.