This is my POV:
Clay is the modern GTM platform. It helps you do everything from enrichment to execution.
No single data provider has 100% coverage, but Clay allows a user to build a logic flow: "First check ZoomInfo. If no email found, check Apollo. If no email found, check another, etc." This "Waterfall" approach maximizes data coverage and accuracy
Apollo is good for enrichment (in the USA - not so good outside) and it is very good for sales teams that need to execute sequences. Clay is just starting with their campaign module, while Apollo has had it for a long time