Of course you need to look professional when you're at work or engaged in some kind of businesslike activity (ie. negotiating with bankers, trying to make a deal with a car salesman, house-hunting, etc.).That is a good philosophy - I am a big believer in function over form, or at least I end up applying it everywhere.
But there's exceptions. When I go to work, for example, I make sure I look good. There's many benefits to doing that. People treat you better, they're friendlier, you command more respect, end up more likely to get noticed by a higher up, etc. It might be stupid, but it works. And since there is that element of function there, I can pat myself on the head and say that function over form principles were not compromised
I went through the hassle of "work clothes" (pantsuits) when I worked at the craft cooperative or at the Nature Centre. I even gave in and got the necessary stuff to be (mostly) in uniform whenever I did front-of-house work at the theatre. At the Arts Centre, that meant white blouse, black pants or skirt, black shoes, and the centre supplied the vest and tie. I never bothered with the tie. Doing other front-of-house work depended on the play - I actually put on a modern dress when ushering for "Grease" (I hate wearing stuff like that). It was much more relaxing to put on my normal medieval outfits when working on the Shakespeare plays.