When I have surplus tech points I change their clothes to things like a white outfit for a doctor so that they're easy to spot (or changing a non-doctor's outfit from white). You can also group your villagers by clothing if you want (say green for farmers, blue for researchers...) Or you can put a special outfit on someone you're grooming for elder. Whatever you want. I never bother until I've everything else paid for with tech points first however.

Oh, and the other time I change their clothes is when my 3 year old helps me. She loves to take them to the "clothes store" and pick out what they should wear.

Tess