The rubber is limp, quiet, and doesn't develop pinhole leaks as much as stiffer materials.
It's quick to look through the side of the bag and see what exactly is in it.
I prefer them to a bulkier laptop case. If I want more padding I wrap a shirt around it.
I made these two bags for a trip to Indonesia a few years ago and have been carrying them ever since. The original plan was to protect my computer and electronics from moisture and humidity. To seal the bags up I rolled the ends up and tied them with a strip of the same rubber they're made of. I brought a kilo of reusable desiccant with me. The wet side of the big volcano in Bali was especially humid. It felt like it was raining off my eyebrows most of the time. I didn't end up spending too much time in super-humid areas and gave my desiccant to people who did.
None of my gear died on that trip, and from there I went on an outrigger canoe sailing trip in New Zealand. These bags were great dry bags for camping gear and food.
Now I carry them with me all the time. I don't bother sealing them, I just fold the ends over unless I expect boat troubles. One bag has my laptop in it and the other has my misc electronics.
I mislaid one of them recently and missed it a lot.
This project is pretty simple, but you can't buy bags like these and they're really nice to have.
Step 1: Cut, Fold, and Weld the Rubber Sheet
Seattle Fabrics sells lots of coated fabrics that would work for this, and they specify which ones can be welded.
Some types of material will need to be washed with soap before you can weld them.
Cut a rectangular piece big enough to make the bag you want.
Fold it over and weld the edge with a hot iron.
Keep it from sticking to the iron by putting a piece of kapton (high temp plastic sheet) or tinfoil over and under it. Experiment with scrap material til you get the iron to the right temperature and get the knack of it. Rub the kapton on your nose to make it greasy and keep it from sticking.
Put a narrow stick under the edge you are welding on to keep from welding too far into the bag, and so you can put more pressure on the edge.
This is the same process (and picture) from Make Your Own Waterproof Camera Enclosure