Really, I don't get it.  What does playing scales over and over gain you that simply playing real works doesn't?

I'm a beginner and I'm currently working on the third movement of the Moonlight Sonata.  Obviously, I can see the benefit of playing C# arpeggios over and over, but I don't understand how playing the entire C# scale up and down will help me.

I'm not trying to stir anything up.  I'm just genuinely curious about the benefits of scales over actually learning scales by playing real works in a given key.