I don't understand how a steroid spray can help chronic rhinitus when those sprays thin the lining and can cause bleeding. how?

Effect vs side effx: The steroid helps quiet the immune / inflammatory response in the sinuses from the irritants and bacteria present. There is a risk of mucosal thinning and subsequent bleeding, but not everyone gets that. It can sometimes be mitigated by using saline nasal spray intermittently as well. Risks far less than oral / systemic steroids.