I like what Liz said.
I shop with a list because I have a budget and can't afford to (frequently, anyway) find some fab new jacket that I have to buy pants, a top, and shoes to go with. I need to know up from what pieces I need to avoid orphaning something I love that I already own. Not that a list automatically means that I know that something will look good with something I own at home. I usually wear "my uniform" when shopping and if the piece doesn't look good with my uniform but think it might go well with a different color or shillouette I already own, I will purchase and try at home.
I'm a big pre-shopper and try to pair types of items together ahead of time then look for one of each type in a pair (i.e. bomber to wear with skirts and trousers I already own, colored denim jacket to wear over thick sweaters with blue jeans). I love YLF finds for this reason-- I'm intensely visual, but not much of a computer person and love to play with the retailer pictures and see how things look together at least on a conceptual level without having to upload or edit a ton of stuff. Since I mostly shop online, this works great.
Plus, I have a clearer idea of what I will actually wear at home, when I'm making my list (i.e. flannel shirts, sweatshirts, and sweaters in neutrals), than when I'm shopping and all I am seeing is cowl-neck short sleeve tunics in vibrant colors. In the past, because of budget, I've given up and bought whatever looked nice that was readily available even if it was far to feminine for my style and made me feel silly after the newness wore off.
I need to go into any purchase knowing I already own something that I can wear with it, or can afford to buy what I want to wear with it. If it is the latter, I keep the tags on until I find its partner.
Plus, I have a pretty good idea about how many outfits I need to get through a time period with grace and a pretty good idea about how many pieces are needed in an outfit during a certain season based on my climate.
I often have "wish-list" items that I've been trying to find for months or years because they fit right in with my style or are practical for me. Sometimes I know I really want to try a trend. Other times I just know I need to buy 3 knit tees and 2 button downs (usually because I've worn out 3 knit tees, needed them and they weren't there, etc.).