Well, the theory is that once our immune system is exposed to a weak version of the virus, it will "remember" and be able to fight that virus off quicker and more effectively next time.
It's the whole reason we have allergies...
Allergies are an overactive immune response. It happens basically because our immune system is fighting off a stimulant way too aggressively, causing us to react badly, this being an allergy.
The theory is great and sound. But what works in theory doesn't always work in practice.
In practice, the virus they use is dead or almost dead.
Problem in practice: the body doesn't find dead or nearly dead viruses as a threat, so no immune response is created.
Solution: use aluminum, that bonds to the virus, to simulate an immune response, because the body find aluminum to be a big threat.
Immunity problem is mostly fixed, still not perfect though.
Problem with the solution: aluminum is a threat that will not filter out, it instead makes is way to the brain, to sit and cause neurological and other damage.
But it happens weeks after the shot. Sometimes after several shots, so it can have "nothing to do with the shots". Studies don't consider those to be causal because the long term adverse reaction studies only extend a few weeks from a single round and everything bad that does happen is just a "coincidence", or "the benefit outweighs the risks" so it's just not worth mentioning ever again.

People just tend to act as though vaccines are only saline and a dead virus. If it was just that, it might be fine. But it's not.

So the theory is sound but the science is manipulated. I've never seen the word "coincidence" with such prevalence in studies on anything else besides vaccines. It's ludacris.

You have a great theory here. Toxoids don't prevent toxins and poisons typically don't prevent poisons, but viruses do prevent viruses. That's why when we get natural chicken pox once, we are immune for life afterwards.

It's the adjuvants that makes it dangerous. That stuff should NOT be injected into anyone, especially babies on the regular!