Rankings show progress. And all teams want to progress. There is no other way to show progress (we can all know in our heads how good teams are, but we need a tool to actually show it). For teams who do not reach the World Cup (Guam, Hong Kong, Papua New Guinea, etc.), this is an important measure of their success/lack of success. Many FAs base their progress reports/funding bids on their position in the FIFA ranking. So they do have importance for the smaller teams at least. Of course, it also helps when seeding teams (previous form in tournaments is not a good measure, as sometimes, some teams have easier groups than others).
And in general, individuals/nations/teams like to know their position in global terms. It's a human trait to be curious about how you fit in the 'bigger picture'. It may not be useful, but it is human.
One could also argue - what is the usefulness of having international football? There is no real usefulness in global terms - the world would still turn without football - but we all like (need?) it. ;-)
Of course, we are all free to be hardliners ;-)