There are a number of applications that use SQL Server as a local repository, and add/remove programs now lists it as such rather than as MSSQL Runtime or just part of the main program that installed it. The runtime version is free for developers to use, and it is actually scalable as opposed to Access, which is the other option, so developers who need a database to support their application will use the runtime version.

Stop the service and see what breaks. That'll tell you why it is installed on yours