Otherwise, what is the use of observing my legs' sensation, when I am fighting with anger, frustration, love, etc.?
When you are angry the distribution of the sensations through the body may take one form. When in love another. There will be places where sensations are more intense, but nevertheless there is sensations throughout.
So why not just focus on ...
Why is it necessary to scan entire body in Vipassana?
You have to understand the full sphere of experience. You have to understand all sensations in the body. If you move from more gross or intense sensations to another, you will never understand the more subtle sensations. You have to come to a state where you experience any sensation in the body, always, and though the body. This is how you will know the whole sphere of experiences is unsatisfactory.
Throughout your body - skin
head and around the heart
Also keep in mind. When something happens in the mind also bodily sensations arise. When something happens in the body metal sensations arise.