I am applying for a scholarship for Masters course. One of the question is "Please give details of why you are applying for this scholarship, providing evidence of academic ability, energy and commitment to studies."

In the details of why I am applying for the scholarship, should I give personal reason or leave the whole personal background out of it?

Personal reasons like you cannot afford paying tuition fees are not relevant. They want you to state your professional plans after study indicating your "academic ability, energy and commitment to studies."

By giving you a scholarship, they are investing in you instead of another person.
I think they want you to explain why they should do that in your case.
Why are you specifically worth investing in? What's so special about you?
eng77's post above gives a good suggestion about how to answer that question.