Why does the Royal Mail have all these weird and wonderful values for
single stamps?

I can't for the life of me see how the stamp values relate to any
usefulness based on the cost of sending a letter. But there must be
some sort of conection like this.

Can anyone work it out?

This page http://tinyurl.com/fpf9u shows the value of individual
stamps I can get from my post office are as follows:

1 pence
2
5
9
10
20
37
42
44
49
50
72

And thse pages is a list of charge bands for different weights of
first or second class post:

http://www.royalmail.com/portal/rm/j...9&catId=400028
http://www.royalmail.com/portal/rm/j...0&catId=400028

What is the connection?