Usually I shop for an item because I need it for a new project I'm working on. That's one category; sometimes that's known in advance, sometimes it comes up during the project. I'll call that a business opportunity buy. It happens fast, but it's part of doing business.
Sometimes a piece of gear in my studio needs to be replaced or upgraded. That's another category. Call it a maintenance buy. That's part of doing business, too.
Another category is long term needs, planned in advance, i.e., I get a piece of equipment or an instrument that I know I can use on lots of projects long-term to give myself more opportunities, and I plan for it..
Finally, there are impulse buys, and maybe in my case that would be if a killer deal comes up on something I've been needing long-term, or a very small item. I don't have a lot of these.