Let's say you've recorded a couple of vocals, a set of drums, two guitars, bass and keyboards. Using only one track for each, that's seven different tracks. (Keys are usually stereo, drums could be seven separate tracks of their own).
Most stereo systems use two tracks, left and right. If we listen on cans, that's about all we need.
First a balance of volume needs to be attained between all the instruments. That can change in any subsequent re-mix. Then a decision as to where in the space between left and right each track goes - pan. It would sound odd if half the track was panned left, the other right. So each track gets its own treatment.
Then there's an amount of eq., which will be different for each track. For instance, if the drums needed brightening up with more top end, without mixing, everything else would become more piercing. Not good. Then there's the question how much reverb? Some of the tracks need none, some lots. Without separate mixing facilities, it would be all or nothing.
There a heck of a lot more involved in mixing, but this is a snapshot. Hope it makes sense.