If it's set up as a business, then people are less likely to ask for little things because they cost money. This can be bad, because it may make people limit their requests. However, it can also be good, because it means there are fewer bogus requests.
If IT is not set up as a business then it's very tricky to determine where time/money/effort should be spent because it's hard to know what requests are important and which ones are just would-be-nice.
There needs to be some sort of accountability between the other areas of the business and IT. One way of doing that is to force the other areas to pay for the work that they want to get done.
If the company is small enough, this can all be done informally. When you've got tens of thousands of people, it becomes a much different ballgame.