It matters because it is easy to sell and hard to margin. Clients like the predictability, and so does your revenue line — but the cost side is only predictable if support volume is, and volume is driven by the client's environment and staff turnover rather than by your effort.
The common trap is treating scope as a formality. Unlimited support for a defined set of systems is a business; unlimited support with no boundary is an open-ended commitment priced on an assumption. The agreements that fail are usually the ones where nobody wrote down what is not included.