The short answer
Lowering LDL a lot can stop plaque growing, shrink it a little, and steady it, so it's less likely to crack and cause a heart attack.
In the GLAGOV imaging trial, people who brought LDL-C down to about 36 mg/dL saw plaque shrink in about 2 of 3 cases. The change is small, and a calmer, firmer plaque is less likely to rupture.
Source: GLAGOV trial, JAMA 2016.