My voice teacher is telling me to learn Piano to get better understanding of music.
I know how to play the guitar & one of my freind knows how to play the Piano.
Is it really compulsory for me to learn it?
Not "compulsory", but very useful.
One reason that every singer should play an instrument is that if they don't, their rhythm tends to suck. Singers don't have to make any kind of gross motor movement when they sing, so it's harder for them to internalize rhythm in their bodies.
The great baritone Thomas Quasthoff ran into that "compulsory" attitude in conservatory. He had to pass piano to get a degree. The problem was, he's a Thalidomide baby and only has one real arm. I don't remember how that all turned out, but it doesn't seem to have hurt his career any!