"Retail therapy" is sometimes used in a joking way by (mainly) women to describe the times they feel like they need to shop. It's sometimes used by others, both men and women, in a sneering way to shame those people and make them feel frivolous and selfish. The implication is that no one needs to go shopping or buy new stuff.
To be fair, there are likely few instances in which something brand new truly needs to be purchased instead of something used or simply making due with what you have at home, but that doesn't mean that shopping for an emotional reason is without merit.
What we often forget is that clothes matter. What we put on our bodies can carry real emotional weight. You don't believe me? Go watch almost any episode of What Not to Wear, How to Look Good Naked, or even go all the way back to Queer Eye for the Straight Guy. What you'll find is men and women truly transformed by being shown that they can look great. And it's sad but true to say that human beings (and lots of animals, for that matter) equate looking good with having worth. So when people who used to hide their bodies or hang their heads are shown that they, too, can preen and shine along with everyone else, it changes them.
We're status-driven. It's a simple fact of human nature. So even if someone goes out and doesn't buy clothes but, instead, something cute for the house, that still can build them up for a while. "Look what I have! Don't I have good taste?" Or maybe you saw something that reminded you of your grandma's house or that just made you happy when you looked at it, that feeling is real.
Obviously, this feeling can become a black hole of addiction; masking other issues that make someone unhappy, but as with most everything else, it's benign in moderation. So if those new shoes put a spring in your step or that funky bird statue makes you smile every time you see it, don't feel shallow. And don't make anyone else feel shallow, either.