Question

Why it is only the difference of potential energy which is significant and actual value at a point is not relevant.
thus there is a freedom in choosing the point where potential energy is zero

Answer 1

Dear student
It is because whenever we are calculating say work done by a force we are interested in change in energy that occurs via work done.if we want to calculate work done by conservative forces than we calculate change in potential energy .If we arbitrarily assign potential for any configuration as zero or of some value than we calculate how much potential energy increases or decreases as a result of work done by conservative forces.
Regards