As I recall, most IQ tests test a person's ability to intuitively recognise patterns in shapes and other visual cues, at speed.
I can imagine that this would be important in football, especially the 'thinking on the spot' part. Most footballers may not possess the academic prowess of us on this forum, but they are able to instantly process tasks that would take a physicists hours, analyzing the patterns in opponents' body movements, computing the likely position of their teammates in the next 0.2 seconds and calculating the speed and trajectory of a ball. All while James Milner is running at them.