I work at a college where a large amount of the college income comes from public funds. We have to purchase things from vendors on state contract to make sure we aren't getting ripped off. (State contract is a joke in itself that doesn't guarantee anything.) We can't just "Go to Fry's," because purchasing nearly everything has a specific procedure to follow.
You had to find companies on state contract that actually sell what you want to buy. Then you had to go to management and get approval for the purchase. You had to fill out a purchase order and submit it to the company. (It basically had to be an emergency purchase to get access to the college credit card.) And then you had to wait for the product to arrive. If things went well, you'd get the item in just under a month. But then sometimes the purchasing department would get a bug up their butt and force you to go to bid anyway. Then you'd need RFPs from a minimum of three different companies, and you'd have to wait for approval for that whole process. If you were lucky, you might get something that went out to bid within (literally) six months after you tried to purchase it.
Not that I'm trying to defend the IT department, sometimes you do just need to go out and get a new hard drive. But it's not always as simple as just going out to the store and buying one.