Now this is not a complete knee exercise it is an excellent exercise to strengthen the lateral hip muscles. These are essential for stabilising your pelvis and for supporting you as you walk so that instead of needing to bend your knee and drag your leg forward using your hip flexors you are able to lift one side of the pelvis and push your leg forward using your glutes and hamstrings.
Your hip muscles need to be able to hold your entire body with every step so need to be very strong. Without this hip strength as you move forward your body may be lurching from side to side or falling forwards with each step.
Strengthening your hips will save your knees.
During this exercise you can observe what your body is doing – is the ankle wobbling? Or are you losing balance? Does your pelvis stick forwards or back? Are your knees bending? Are your arms getting involved to help you balance?
These are signs that you are using these parts of your body to compensate for weak hip muscles – the answer is to do this exercise more often and build up the amount of time you hold the pose.