I can actually take it from both perspectives.
For a long time I shopped the Woman's Department, I mean as I graduated from college in a 3X top. I wore the young fat and frumpy thing quite well.
I'm also a Department Store Merchandise Coordinator. My job is to know my merchandise well enough that I can train the sales staff to sell it to most customers. Ask me a question about any of my regular brands and I can tell you exactly how they fit and how well they hold up to wear, tear, and the washing machine. When I'm training on a brand, I make sure the better sales associates know exactly what they need to know about the fit of the brand.
A couple of weeks ago, my store manager decides to be the PIA that she can be and reorganize everyone's responsibilities in the store. Thus I find myself now reading style guides, floor plans, and all the other fun stuff that shoppers don't have any clue about for the Women's Department.
Fast Forward a few days, I walking the Women's Department with the sales manager, who's known me to never be bigger than a 14-16, which is 55% of our Ready to Wear Business. He makes a comment on how well the Caribbean Joe Women's line has been selling, and it had to be because of the way I set it so well. I come back with I just remember what it was like having no luck finding anything fashionable up here. He looks at me with a raised eyebrow and goes "What do you mean you actually shopped up here?" I assure him that I spent many of years shopping the department, and I am determined to drive the numbers so the non-Frump stuff sells, so the buyers send more and we get the sales numbers back up.
Bottom line is, good sales staff is going to know anything they possibly can about a brand. There are brands that run small and brands that run big. If they don't know the differences, that's a customer lost because of dissatisfaction.