I definitely get what you mean; unless you've done it, it's realllyy hard to understand why it would help. It does actually have a scientific, real reason behind it though. When someone cuts themselves, their brain reacts to the wound and the pain by releasing endorphins. These endorphins immediately bring a calming affect, and it makes them feel almost happy. They are the same chemicals that are released when you exercise; they are your brains way of handling pain or stress to the body and they literally calm you down if you're upset.
The problem is, these chemicals are actually addicting. A cutter will eventually realize that they don't want to cut anymore or they will get scared because they keep cutting deeper(just like any addiction, the brain eventually keeps needing more and more of the source to keep the addiction under control), but by that time, it is too late. If they try to stop right away, the moment they get upset at all they will experience actual withdrawal symptoms and may end up in a sort of panic. So, you are right; it does cause them more stress. At the same time however, they become dependent on those chemicals and they don't know how to handle stress without them. Most cutters don't actually enjoy the literal cutting part. They just get hooked to the endorphins.
Another way to look at it is like this: If a child is arguing with their sibling and they are upset and angry at them, but then they fall off a bike, what are they going to think about? They will automatically stop thinking about the argument and start crying, immediately focused on the pain from the fall. That's like cutting. They need something else to focus on to get their minds off stress or sadness, so they focus on the pain.
I really hope this helps!!
Former cutter...also wrote in depth research papers in college about cutting