This is a good question. I'm going through my closet as well. I, too, have things I wonder why I bought!
I do keep things I haven't worn in a while, some I shouldn't, but most others because they are for certain occasions or certain weathers. For example, I have heavy jackets and clothing I don't wear much here in CA, but my brother lives in Boston so I definitely need them. I also have suits I don't wear too much anymore due to a change in work, but keep them because I know I will need them, especially now. I think these kinds of things are exceptions to the "6 month rule" - anything weather related in particular.
I don't really keep too many things because they may come back in style. I think I usual buy pretty classic iterations of trends, perhaps. I think my biggest problems is weeding out shoes and purses. Over the years I have spent a lot of money on both and really have a hard time weeding them out.
The thing I have found, though, that when I weed out pretty ruthlessly, is that I love my wardrobe more. I find it easier to pull together outfits I love, etc. Oddly it makes me feel like I have more!
I'm sure after your shopping experience with Angie that what makes the cut has changed. Good for you! I can't wait to shop with her someday soon.