Recruits your core muscles
When doing high knees your body will require assistance from your core or abdominal muscles. This way the exercise is more effective, but not only that, activating the core muscles can also help improve posture.
According to research from 2015, running in place, which is similar to high knees, while also contracting the abdominal muscles, can help improve posture.
Basically, the high knees exercise improves cardiovascular endurance [3], burns calories, boosts lower-body endurance and strength, improves coordination, and strengthens the abdominal muscles.
Targets your lower body
High knees is also a very effective exercise to help build your lower body. They help activate your quadriceps, hamstrings, calves, glutes, and hip flexors, helping strengthen your muscles and improve muscular endurance, muscular balance, and coordination in these muscles.
Also, when you perform high knees with high intensity and with bounding or explosive knee drives, they have proven to help improve power in your lower body.