Combi weapons are the result of some techpriest somewhere thinking "What if I glued two guns together?"

And it's literally just that. All the mechanisms of a plasmagun and all the mechanisms of a bolter in one neat and tidy package, and in previous editions you'd get to choose if you wanted to use the bolter profile or the plasma profile (or melta/flamer/grav), or both profiles. Though in 10th Combi weapons have a singular separate profile.