Virtually everything on a webservice will be provided or managed by a database. Web pages are "constructed" from information provided by the database and arranged on web pages, controlled by the database, working with a "business layer" of code.
This needs to be fast, accurtate, secure and reliable. Building on the soid base that is the database, the web service platform must be coded to efficiently query and work with the database. As part of the Microsoft family, SQL Server is closely integrated with the development languages and tools provided by Microsoft. Fast and efficient.