Probably the most common topics that parish councils get involved with are planning matters (they are statutory consultees), crime prevention and roads & highways.
It’s true to say that on their own, parish councils have limited powers to make decisions (other than those listed on the Powers & duties page (of South Norfolk’s website). But they do have the ability to negotiate with, and the power to influence, those other organisations that do make the final decisions (such as the district or county council, health authorities, police etc.).
In this respect parish councils are extremely powerful. The organisations that make the final decisions know that a parish council gives the best reflection of how a community feels about something, and it’s views will be taken seriously.