Objects is a fourth for me, because I love making my homes beautiful. Hair is a third for the simple fact that I love the styling of custom content hair and that it gives a more realistic look to the sims, and makes each of them more attractive. Males are too limited and just provide a plain shirt and some random jeans/sometimes shorts.
Especially, because maxis clothes are so basic and for females never fit the figure right. Clothes are a close second, because I would absolutely hate to see my sims without cc clothes. And each of my creations has a unique look.
it gives a more interesting and dramatic flare to how my sims look. I like to use the skin details custom content the most.