About boss wanting me to fill out a FMLA form, when I have been on workmans comp.?

I hurt my hand at work and have been on Workman's comp. But still working at my workplace doing other jobs that I can do. So I am getting paid. They want me to fill out a FM LA form. I am wondering WHY? Is there something that will change there responsibility for me, if I fill out and sign this form? I have read about it in my workbook about this act, Something tells me that things are just Not Quite Right! But I really would like to know just what they have planned. I was told by them that it is to protect me. From what I have read, it doesn't show me that it is true. But shouldn't they still be responsible if this form was signed? Why is it so important to them that I sign it?