Why do some models have the head and other parts separate?

[smokingSnakeRIT]

I'm new to 3D modeling and would like to know the reason for this. Does this make the model lighter for animation and for use in games?

[Serious]

While i'm also pretty new to moddeling i made lots of maps/mods for different games and i think the main reason is the reuse of the body parts.
You can easily use the same body scaled with a few reasonable numbers for 100 and more people. But you can't, or at least you shouldn't use the same head and face again and again.
You can also seperate the animation for faces and bodies easier this way, like a person could sit on a chair but with many different face animations. It would be also possible as one mesh, but it's easier as body/head.
A further seperation of the body on the other side could increase the complexity of animations again as most realistic movements are not just one hand or leg.

There was also a time where a highly detailed body/head could break some limits e.g. 64k vertices but i think with todays processing power this is rather unimportant, except you have to export your model to an older program or engine.

[*P0P$*FR3$H3NM3Y3R*]

I'd guess that it's probably not much of a performance thing, but easier to animate as you don't run the risks that vertex weights influence torso, legs etc.. And swapping stuff on the fly for stuff like damage effects, poppin heads, adding variety (like prev post).
Like a Lego Minifig where you can quickly swap and generate different iterations fromm some building blocks.

[mr munkee]

It basically comes down to the fact that everything is purpose-built. When I was new I had a lot of questions like this. Over time after having more experience you just sort of realize things or understand why something might be how it is. It's hard to explain sometimes, but if given specific examples then it could be easier to answer the reasons for certain things.