It is a tool to stretch webbing on the bottom of upholstered chairs.
https://www.amazon.com/Osborne-253-Gooseneck-Stretcher-Upholstery/dp/B002U2RADQ
I use these to tack a piece of fabric for gridding (so I don't have to keep moving it around on a smaller frame/hoop). I don't use the bars for stitching though, I'm not sure how well it would work for stitching as the corners aren't too secure. They slot together but they move fairly easily. You can buy a small set and try it, they're not very expensive, I just bought this set so I'd have a variety of sizes and can combine them to make different sizes. https://www.amazon.com/dp/B07D8GRLML/ref=cm_sw_r_cp_apa_fabc_EXMD0M32RQMXN0VYR2J1
Got them from amazon. I wouldn't necessarily recommend them tho. I ordered, not knowing it'd be coming from China, took a month to get here (of course) but 3 out of the 4 were so warped and not flat that I couldn't work with them. Contacted the seller, sent a few pictures and they sent me 3 new ones free of charge (waiting another month of course). Can't complain about the customer service at all. Just hate that it's been 2 months since I've sealed it and it's just been laying under my bed and I was looking for bars specifically that wouldn't come from China.. I might have just not looked hard enough.
The way I learned to do it was with gummed paper tape, but I've not found a brand recently that holds as well as the tape I used to use back then. Nowadays I tape it to get a clean edge on the art but then staple it with a staple gun to do the work of holding it in place.
The best way to protect the edges is to not try. Just use a bigger piece of paper than what you want your final piece to be and then just trim the edges off when you're done.
Alternately, you can use a watercolor stretcher like this one, use heavier paper or watercolor board, or try using a watercolor block (which I'm not really a fan of).
On Amazon 30 by 40 precut stretcher strips are about $15usd, plus shipping.
You can find them similarly priced in art/craft/hobby stores here in the states. Or you can pickup some 1" x 2" strips of lumber and some 1/4 round from the hardware store and you could probably build one for about $3-4 each. Maybe less depending on where you live.
The same with the pliers, but it might be nice to hold the pair in your hand before you buy one if you plan on using them a lot. They cost anywhere from $15 for a cheap pair to $125+ for a more "pro" model. My advice is to not skimp here--you probably won't need the $100 model, but some easy to grip, ergonomic handles are really nice to have.