There is evidence that 20mph zones do save lives and injuries even if people don’t obey them. This is because they still drive slower than they would in a 30mph zone.
I would agree, however, that if the limit is set to 20mph then the road design needs to be changed to match that, making it uncomfortable for drivers to exceed the limit. Unfortunately the UK is quite institutionally poor at this kind of traffic calming design compared to some of its neighbours.
Sets are super useful for all sorts of stuff where you want the datatype to guarantee there is only one instance of a value in the collection. UI components where you are incrementally adding things to a selection is a great example.
It’s a completely standard feature in many stdlibs and has nothing to with turning JS into a database.