The reason new exists is not just the styles, but the higher likelihood you are going to find the correct (or near to it) size.
Also the cost factor is nice. My partner works at a local thrift and getting steep discounts on top of the already reduced prices is amazing. And whenever something outgrows my wardrobe it's back off to the donation bin (wear providing)
Sizing is tough though, you're right. My body is outside being sized for most clothes so a ton of otherwise good finds just won't do (but that's what traditional shops are for filling). Thankfully my partner has a similar sizer torso so we can share tops, jackets and whatnot style providing. Organizing two stuffed closets is a bit tricky however
There's always more stuff to dig through and finds to discover!
disclaimer: I work for Curtsy.