I think the big difference is that they come with the OSes that those types of people are likely to use. And being stores, they provide software that package managers are unlikely to provide.

I think package managers becomes less useful to average users when you can't expect all of your software to be there.

The only thing I can think of is that they don't really install OS level stuff, think software libraries or runtimes etc.

Other than that they're literally identical to something like synaptic.