Tight Hip Flexors affect Athletes as well as the general population. In Evolutionary terms humans have not been standing fully upright long enough to have perfectly adapted hip flexors. With the amount people sit nowadays (driving, couch, computer, etc) our hip flexors are consistently closed allowing them to shorten. They almost never receive any kind of stretching unless your a very conscious athlete or rehabbing from injury.
When the Hip Flexors tighten the Pelvis is put into a tilt partially disengaging the Glute complex which is a major stabilizing group. This affects the arches of the feet as well as strains the Hamstrings and puts the back in Hyperextension. Injuries can occur including ACL, low back, and Piriformis issues. Other issues that Athletes should be aware of is while your body mechanics are altered due to the torso Hyperextending the Diaphragm is in a partial inhalation position that affects breathing which will alter your endurance.
To prevent adaptive shorting of the Hip Flexors include stretching plus corrective exercises to the Glutes, Hamstrings and Adductors - which are Posterior to Hip Flexor function.