Hi,
Well, the idea is to lock in the higher rates associated with longer term CDs, but giving you flexibility by laddering them, so some of your money will be liquid every year.
However, with the way rates are right now, you don't get much extra interest from a 5 year CD over what you can get at ING or similar high-interest savings accounts, so I think some of the "benefit" is not that strong right now.
Also, CDs issued by a bank are FDIC-insured, whereas MMs or other such accounts may not be, depending on where they are housed.
I'm sure someone here can come up with a much more intelligent answer, but it's a start...
Karen