Your Sim's head is permanent. Can't change heads like we can change clothes, and while that is limiting, at the same time it's not. What do I mean? I will tell you.

Take a look at this head file for the headless horseman( availible in 7DS's wrath):
// Character File. Copyright 1997, Maxis Inc.
version 300
0
1
c529ma_headless
0
0
3
HEAD
xskin-c529ma_headless-HEAD-HEAD
0
0
R_ARM2
xskin-b529mafit_horseman-R-ARM2-HHORSEMAN
0
0
L_ARM2
xskin-b529mafit_horseman-L-ARM2-HHORSEMAN
0
0
0

Ok it's not very dramatic, but I'm sure you notice that there are some body parts in this head file. What does that mean? It means he's wearing big gloves with cuffs ALL THE TIME. Not just when he's in his bad-@ss armor, but when he's sleeping, when he's bathing, when he's wearing his tux.

Why did I do that? Because when you make gloves, they are permanent with that particular outfit. I figured he'd look lame with black hands and his pink banana hammock o a swimsuit, so I fixed it so he's wearing the full gloves all the time.

But that's just a mundane reason. If you're customizing, you can do it for any reason. Say you made a She-lion. Where would her tail normally go away when she's in PJ's in the game? It would vanish, and just how real is that? It's not. So implement her tail .skn file in the head .cmx and ta-daaa! Her tail is always with her. Same if you made a demoness with wings. I'm so sure she's going to stuff them in her Pajama top. Put them in her head .cmx and they will always be there.

The possibilities are endless.

The same goes for outfits as well. Joe Reaper (in Wrath, and the reason I figured all this out in the first place) has a changeable costume, so to speak. Yes the hood attaches to his head, but how dumb would it look for him to be in his swim shorts with a hood still on his head? All you do is add the head attachment to the body .cmx file. With this, you can make anyone wear the robe, even your granny.

There is alot you can do and alot you can make. Your imagination is the limit. I figured out all this by just wondering "what if I....." and you can to.