VCA (voltage-controlled amplifier) faders don't pass audio. Rather, they act as 'remote controls' for other faders: assign a track fader to a VCA fader, and now the VCA can control the track's gain, without moving the associated track fader. Assign multiple track faders to the VCA, and you can bring all their levels up and down simultaneously. You can also adjust track faders independently of a group controlled by a VCA fader, without causing the other faders to follow along.
Cakewalk doesn't have VCA faders, and it's understandable if your response is, "What's the big deal? Can't you just send the channels to a bus, and use the bus fader to control the level?" The answer is yes, and if that's all you need to do, groups are fine. Even with a channel in a group, you can still adjust a fader independently of the other tracks in it by holding down the Ctrl key while moving the fader. When you release the fader, the control returns to being part of its original group.
However, there are other VCA fader functions that Cakewalk can't do, like having the same fader belong to different groups. This can be helpful for projects with a high track count. For example, you could have fader groups for violins, violas, and cellos — nothing unusual there — but you could also have a violins + violas fader group, and yet another fader group with all the strings.
One of the classic scenarios where VCA faders win out is when several tracks (say, individual drums) go to a submix fader, and the tracks also have post-fader send controls going to an effect such as reverb. With a conventional submix bus, as you pull down the bus fader, the...