I've used them as a bridge to the shore. Never found another use for them.

The larger ones I use to reach the shore without having to park right up against it.
The shorter ones I typically use as trapdoors.
E.g.
It's a little hard to see as the toolbar is in the way of one and the other is on the other side of the pillar.
https://steamcommunity.com/sharedfiles/filedetails/?id=3272107821
This one you'll need to zoom in, it's on the far right.
https://steamcommunity.com/sharedfiles/filedetails/?id=3201066545

There's other examples of this on that ship, but I don't have images of it. Anytime I had a pile coming down from the ceiling or up from the floor I had one of the shorter ones covering over the hole.
I also had one over the hole I used to drop down from the living area to the working area (if I was too lazy to use the stairs).

As for making a second part of the raft? They don't need to be connected and you could always just have a "make do" bridge until you can get the drawbridges.
(Not an example of drawbridges, at least not one that can be easily seen, just to show you can make a 'multi-platform raft'.)
https://steamcommunity.com/sharedfiles/filedetails/?id=3139368434

I believe it's useful when you get new animals from island to raft

it came in handy for this raft. i was at 99 floating tiles (like to keep it 100 or lower so only need 1 engine) and used it to access the 100th tile that i put that big shark on. other than that it's pretty much for looks. used the smaller one as a walk the plank thingy for my pirate ship :)

https://steamcommunity.com/sharedfiles/filedetails/?id=2915300962