Why do enum permissions often have 0, 1, 2, 4 valu

Question

Why are people always using enum values like 0, 1, 2, 4, 8 and not 0, 1, 2, 3, 4?

Has this something to do with bit operations, etc.?

I would really appreciate a small sample snippet on how this is used correctly :)

[Flags]
public enum Permissions
{
    None   = 0,
    Read   = 1,
    Write  = 2,
    Delete = 4
}

Answer 1

This is really more of a comment, but since that wouldn't support formatting, I just wanted to include a method I've employed for setting up flag enumerations:

[Flags]
public enum FlagTest
{
    None = 0,
    Read = 1,
    Write = Read * 2,
    Delete = Write * 2,
    ReadWrite = Read|Write
}

I find this approach especially helpful during development in the case where you like to maintain your flags in alphabetical order. If you determine you need to add a new flag value, you can just insert it alphabetically and the only value you have to change is the one it now precedes.

Note, however, that once a solution is published to any production system (especially if the enum is exposed without a tight coupling, such as over a web service), then it is highly advisable against changing any existing value within the enum.