How-to documents and recipe books are great and have their places, but so does a compac little reference like this.
iptables rulesets are the sort of thing that you normally set up once and then forget - use it or lose it. So, when the time comes to make a change to your rule set, just reach for this little gem to help you remember the syntax, features, etc.
I look at this pocket reference like insurance: It's better to have it and not need it than to need it and not have it - but if you have trouble sleeping some night and want something to do, you might actually learn something new by browsing the pages.
Another useful addition to my work area as I contemplate translating my ipchains rulesets over to iptables.