The Truth About Preamps

I'm wondering now that we're in the digital age with so many cd players having variable outs, what does a high quality preamp do for a system? I've heard so many different stories on either you get more detail straight in or that you get more bloom with a preamp. I would think that if you had a musical cd player such as a Cary running straight into a musical and smooth amp such as a Classe or Conrad-johnson you wouldn't need a preamp...all this given you have only cd as your source...I'm just wondering what you guys think and your experience running a cd player straight into an amp. I'm thinking of running a Theta Miles straight into a Classe CA-300