Because it keeps my hair clean...or at least no one can tell whether it's clean or not.
This is a small but important part of living simply and conserving resources. I don't have to wash my hair as often--saves time and hot water and shampoo. I don't have to dry it as often--saves energy and time.
In my way of life--farming, or sometimes just the fact of having short-term visitors stay with me--there is also the avoidance of "ick factor" to consider. As demonstrated today:
Since I'm transitioning to a much smaller home this year, I'm swapping big furniture for small with my daughter. They brought the bed over to switch out today. Well! Obviously I hadn't vacuumed under the heavy bed for a long time--GROSS! Off I trotted to get the shop vac out of the garage to eradicate the dust dragons (they were WAY beyond bunnies) before installing the new bed.
It sounded really strange. I mean, not like I've ever heard a shop vac sound before. Dust started spouting from the exhaust port--wait, no, that's not right; I have one of those wonderful ultra-fine filters on this!
I stopped and unplugged it and peeked in. EWW! It was fully of pennies and cigarette butts (a visitor had casually mentioned cleaning out a friend's car a few days ago...), and the filter had fallen off so it had been venting that foul blend at me...plus the garage floor residue that I'd vacuumed last week. ICK!
I took the filter out to the barn, where there is an air compressor and a lot of fresh air, and blew out the filter. That worked really well to revive the filter, but then I looking like a close relative of the dragons under the bed. YUCK!
By the time the bed exchange had been completed, it was less than half an hour before I was supposed to be at a meeting in town. There certainly wasn't time to wash and dry my hair--there was barely time to jump in the shower. But quickly tying on a clean covering let me feel like I wouldn't offend the people next to me at the meeting.
At least not with the cobwebs, dog hair, and cigarette butt dust in my hair.