This is a little bit of a mystery. I don't know, but it appears that all 5 brackets are the same, and my assumption is the two central brackets are for a removable rudder. So, if that is true, then part of the mystery is that a previous owner probably installed all 5 brackets at about the same time. (That is making some assumptions, but just going on what little we have to go on.)
So, for me, part of the question is why the original rudder mounts were replaced and how it was that this coincided with the motivation to install the other three. Maybe it was just coincidence or the owner decided that, as long as they had to do one thing, they might as well do the other thing at the same time.
It is an odd choice of hardware for a motor mount, but I like the hypothesis that it is more for an electric trolling motor or similar. It is just too lightweight for anything but a very small motor. I would also be inclined to think along the lines of a swim ladder/platform, but my hunch is that isn't it.
My approach to these sorts of mysteries is to look carefully at wear patterns, both on the brackets themselves and the surrounding area. You can't use anything without leaving some evidence as to how it was used (little scratches, scuffs, residue, etc.)