I agree, nothing beats trying clothes on. Although it does help to have some idea of what size(s) to take into the dressing room!
More to the point, though, I don't have the courage (or the self confidence) to go out dressed in public, or to try on dresses in a store. And I suspect I'm not the only person with that problem.
And yes, I know that's more my hangup than society's hangup, at least these days. But no matter what I tell myself, I can't get over my fear of being publicly humiliated & embarrassed (and let's face it, that is possible). And I know that if I project that attitude, I'm just asking for trouble! I'll bring out the worst in people.
On the positive side, I have managed to learn how to project a confident attitude when I shop for my (*cough*) wife/girlfriend/daughter/whatever. I've never gotten "clocked" -- not even in a bridal store. Or if I was, the clerks were very polite to me, and saved their snide comments for each other, after I left.