What the difference between using a cock ring or a simple rubber / elastic band? Won’t a rubber band do just as well?
Well, rubber bands, and elastic hair ties for that matter, have been used, but they have their up and down sides. On the plus side: there it is, and the elastic pressure will help to hold blood inside of the penis. They can also hold a slippery external condom in place.
However, A rubber band is not adjustable, and also can cut into the skin in an uncomfortable way. Most commercial erection rings have rounded edges, or wider bands so that they don’t cut into the skin and cause irritation or skin injury. And many commercial rings give you the option of easily adjusting the fit if you wish to do so. You can also get commercial rings in a variety of materials, so if you are allergic to rubber, you can still use a cock ring made out of elastomer or silicone. Your penetration partner might not appreciate the extra sensation, either.
If you’re on a budget, a wide rubber band may work fine as long as you or a penetration partner aren’t allergic to latex. But watch for skin tears and chafing, and change to a commercial product if you have any of these start to occur.