How to answer the question "Why do you deserve a scholarship?" when applying for a scholarship?
Committee must be convinced that the money will be well used. It would look best if you told you would invest the money to become better at the field of your studies (buy literature, go to conferences, pay for some courses or certifications, publishing your articles in journals). However, trying to look perfect could backfire. In my country it would be reasonable if you wrote that part of the scholarship would be spent on something not related to education. But that depends on the country, culture, traditions, committee members etc.

You must also convince the committee that you will do good in studies. That is why you should mention your study results and achievements.

You must convince the committee that you like the field of your studies, you are passionate and will keep working hard. If your studies have helped you at your work, mention that. Also mention your short and long term goals - educational and professional.

You can also mention your strongest characteristics and give arguments. (example: leadership abilities, you lead a student team in project x which was successful. )

The motivation letter as a whole is the answer to the question "Why do you deserve a scholarship?”

edit #1: I changed the part about spending scholarship money on something not education related, because of the comment by Trylks.