Yeah, with DJing or a band playing live, you want a mixer to gather all the things making sound in one place, and combine them to send to the speakers.
But as I was saying with multi-track recording you don't want the pieces combined until the final mix-down. That's actually the same as live, but then then mix-down is happening "right now", were as with a recording the final mix isn't done until later.
Even if you were to record a DJ set for later release, it would still be advantageous to record each deck and the effects on individual tracks, so you could make tweaks to levels and stuff. So if you took a computer with a multi-channel audio interface instead of a mixer, you could do the mixing live in the box.
