Motivation Letter for a particular industry

Hi,

I was recently told by the HR to write motivational letter to apply for a position in their IBD team. I have recently attached cover letter, and hence I am rather confuse with why the need for motivational letter.

In addition, what are the possible reasons I can give if I am interested in a particular industry? For instance, if I would like to focus on F&B industry, because that's where my interest are (as I like food) would that be a ridiculous reason?