Why do runners or those who love to run need to strength train? For one, because of movement impairments that occur due to altered muscle performance. Your gluteus maximus and piriformis are dominant hip extensors because of their attachments (greater trochanter for Glute Max) which allow for greater control of the femoral head in the acetabulum (hip joint). It also is attached down on tibia, via the IT band (lower leg bone) which means that it is significantly important in its function of keeping the femoral head in the hip joint.
However, most distance runners have been associated with weak glutes and psoas (hip flexor) muscles. This causes our TFL (other hip flexor), rectus femoris (hip flexor/knee extensor), and hamstrings (result from lack of glute contribution) to become dominant. Consequently this results in a lack of balance between our hip flexors and extensors=movement impairment. The hamstrings (semimembranosus, semitendinosus) do not attach on the femur except the biceps femoris (which attaches further down on the femur). So think about it, overactive hamstrings cannot control the femoral head when in hip extension, thus a factor in why runners can be getting hamstring & anterior hip pain.
To illustrate this example, a prone hip extension test from Sahrmann’s book. The top photo is when the glutes and psoas are functioning properly (notice the close attachment of the glute and psoas can control the position of the femoral head. The below photo is showing hamstring dominance/weak psoas contribution (notice NO close attachment on the femur and the arrow demonstrating an anterior glide of the femoral head in the hip joint capsule)
These are just some activations for the psoas and glutes that can be incorporated into a program to limit these movement impairments.
Psoas Activations (Hips must be flexed above 90 degrees without any lumbar flexion for the psoas to fire properly)