[bradfa]

As my sibling comment states, mandatory spending is spending which is required by existing laws which have already been passed. But a secondary point is that mandatory spending means no one knows how much it'll be ahead of time. You can only find out how much actual mandatory spending happened after the fact. Contrast this with discretionary spending, where the budget says how much can be spent on a given set of things (like airplanes, or road improvements, or new toilets for all the Navy submarines) so you know ahead of time the spending amount.

Debating changing mandatory spending laws in order to hopefully reduce mandatory spending is totally allowed, it's just very hard to know exactly how much you'd save with any change. The CBO will make estimates for you, but those are going to be based on assumptions which may or may not turn out to reflect reality in the future.