Eg., suppose a pipe breaks, you search for plumber, you find a plumber. That search is a lead generator for that plumber.
That search is "free", by intuition, kinda destroys our sense of value in lead generation, right? Google should NOT be free for that plumber; the plumber should be paying quite a lot for the service.
Suppliers of services need a system which allocates demand, right? Indeed, customers need a system of finding suppliers.