Pubmed is essentially Google for scientists. Anytime you search for scientific publications you usually use Pubmed. Of course there are alternatives, but until now you didn't really have to know about those. Everyone just used Pubmed, I'd bet that even most European scientists didn't know the local alternatives until now.
And there's a lot more functionality made available to scientists by the NIH.
I almost never use pubmed, while I use Google Scholar everyday (for neuroscience/medical/computer science research). But I admit that all medical researchers I know only use pubmed.
And there's a lot more functionality made available to scientists by the NIH.