Would you go to a friend's wedding in crumpled clothing? Or a tracksuit? No. Because it shows you don't respect her. Or yourself.
Same goes for looking like a slattern anywhere else. I'm not saying I look amazing all the time, I don't. But I am clean and tidy. Our of respect for myself, my colleagues, my friends or whoever I come across.
Also: it's not good for your self esteem to look like you're depressed. Which is the vibe given off by folk who clearly haven't made any effort at all.