This is a great thing to have if you are doing quilting and want to keep your sewing lines evenly spaced.
There are other applications as well.
I tested it on all 3 style feet, Old, New, and Classic and it fits into all the different style feet that accept the guide and have a screw to hold it into place.

Someone wondered if it would rest level on her newer 750 QEE machine.  I put it on my B780 and it seems to work fine.  I added 3 pictures of this as well.