1. Someone who wants only one feature that's not in self-service but doesn't have the budget to pay more. 2. Someone willing to pay more but not enough more. 3. Someone who's just trying to compare prices between vendors but isn't a serious buyer (especially when not an existing customer). 4. Various non-obvious spam (affiliate resellers, etc.). 5. Someone who's interested but can't get a sale to happen given their role: has no budget and their manager doesn't have budget, has no authority on technical decisions. 6. Someone who's only curious. 7. Etc. etc. etc.

The level of effort required to press that button is very low, so it isn't that great of a filter. Again, they take those calls because the lack of filtering doesn't mean it's never a quality lead.