A Site Pages Library is created by default with every single SharePoint team site. It holds the wiki page that makes up the home page of the team site and creates a How To page as well. (Don’t forget to always delete that how to page so it doesn’t mess up your search results). A default Site Page (Home) is embedded into a Team Site. You can make additional site pages to display information that seldom changes or doesn’t need to be emailed out.
A Pages Library is created once you activate the Publishing features, which we usually do to get the Navigation link.
The Pages Library is used to store custom brand pages, as well as article and news page layouts.
Now you know. 🙂