I guess the intuition is that the human immune system is an incredibly sophisticated, evolving capability that over time, and over the mass population, can respond to become resilient against a wide array of pathogens.
So if you could somehow capture the best of that (like, for illustration, take the immune system of the healthiest, most resilient person, 30 generations from now in the future) and convey that benefit to many other people, that would be truly awesome.
Sort of like an idea of training an AI, more iterations are better, the immune system of any one person at any one point in time, might not be so great, but the best immune system of the population after encountering many pathogens, is going to be really great.
How can we short circuit that "natural training" process and enhance the immune system? But I agree that attempts to do so might run the risk of creating or exacerbating autoimmune diseases...But I still think it's worth it.