Off the top of my head, I can think of 3 reasons.
1) Squats and deads use a very large amount of the muscle mass in one's body, causing a release of growth-inducing hormones, which can heop gains in the whole body.
2) The body doesn't like to be unbalanced. If one muscle group is lacking, the body can prevent stronger muscle groups from developing further in order to protect itself from injury. Therefore, bench press progress can stall if the legs are neglected.
3) More strength in the core muscles (abs, obliques, lower back) can give one a tighter base on the bench. They can also help one transfer more energy from the leg drive.