@scyonx: UseSqlServer is telling EF that you are targeting SQL Server for that particular context instance. AddSqlServer is about registering services with the global ServiceProvider. If you had two contexts - one targeting SQL Server and the other SQLite - you would add the services for both providers in Startup.cs but only configure the appropriate one to be used by each context.
While you can use ServiceProvider in other types of applications, the pattern will mostly just be used in ASP.NET 5. EF works just fine without creating an external ServiceProvider and passing it in. You can just create a context, call UseSqlServer in OnConfiguring and EF will take care of creating the required services internally. But if you did this in ASP.NET 5 you would loose global logging etc.