The geoip iptables extension allows you to filter, nat or mangle packets based on the country’s source or destination. This does exactly what the geoip apache module does, or the regular geoip binary, but at the iptables level. More here
I would not go into the details why you would want to use that, but there are many ‘positive’ ways it can be useful… For example myself I use it in a project where we want to serve customized content for different countries. Since this is a high traffic site running on many web servers behind a loadbalanced setup, we prefer to split this at the loadbalancer level and not at apache level, to simplify our setup. We serve customized content to the US based visitors, while for the other countries we serve another international site.