It's okay to not understand what 'floats someone else's boat'. Variety is the spice of life and we are all individuals.
If you truly want to understand why someone would want used panties...the answers are in easy reach. You need to only open your mind and ask the questions...as it seems you've done here.
On the other hand, if your post is more about looking for support in your judgment of others...then you've gotten that too. It can make some people feel better to call others weirdo's or fucked up for liking what they like. Cuz if 'they' are the fucked up ones, that makes you 'normal' and you can sleep better.
The truth is, all kinds of things turn on all kinds of folks. As long as it doesn't harm anyone, it's all good.
There's a whole lotta people interested in used garments. They might like to sniff them, lick them or jerk off with them. Maybe the thought that it's naughty and some people think they're perverts for liking it makes it even hotter for them. Shame can be a powerful aphrodisiac.
I licked pussy about an hour ago and can still slightly smell it on my face. I like it. I wish she had left her used panties...