How do I get a consistent byte representation of strings?

How do I convert a String to a byte in .NET (C#) without manually specifying a specific encoding?

 

I'm going to encrypt the string. I can encrypt it without converting, but I'd still like to know why encoding comes to play here.

 

Also, why should encoding be taken into consideration? Can't I simply get what bytes the string has been stored in? Why is there a dependency on character encodings?