I have only read a few studies about it, none related to crohn's disease. It's not because it eats other bacteria or because it helps the immune system that it works.
Once they infect people the hookworm become a primary antigen and will invoke a strong response, what happens after that I don't really know fully. It has a strategy to survive since it stays alive in people for long periods, it's modulating the immune response and causing apoptosis of T cells, both killer and helper cells I believe.
What I don't think it's doing at all is "boosting" or helping the immune system, that would be lovely, if it was actually increasing lymphocyte response you'd give it to aids patients, that doesn't happen. It's a type of bacteria that is an immunomodulator and that's how it tries to survive, if anything it's doing the opposite instead of helping the immune system, it actively targets lymphocyte.
In fact, regarding HIV, they actually are testing how hookworms work and their effects on people to study how the HIV virus works, because hookworms manage to deplete the T cell response, I think the last thing you would give an HIV+ person is a hookworm, it doesn't help the immune system it's a type of immunosupressor and it's depletion of T cell is how it's surviving, and to me that's how I think some people with crohn's disease feel better, since it's actively surppressing the inflammation by suppression of T cells. If it was helping T cells in any way you'd give buckets of it to AIDS patients or people with PID, no one is using this for PID diseases.
In Africa people with HIV get infected with these hookworms, it makes the viral infection worse not better.