For example, if you have the following forums set up and number them 1 to 5:
If you add a forum in the future between forums 1 and 2, you will have to renumber every forum below it to get the order correct.
If however you number them 10, 20, 30, 40 and 50 then you have the ability to add 9 more forums between each one.
If you want to be really on the safe side then you can number them 100, 200, 300....
The same principle applies to display styling priority for user groups.