There are times of the year, and races of bees that will need an entrance reducer to stop robbing, keep cold air out, keep rats and mice out, and a whole myriad of other things.
There are times when a hive is a bit weak, or if it is cold when it can be useful to reduce the size of the entrance. This is what an entrance reducer is for.
There is no one size fits all entrance reducer, or answer to how to make them or the size they should be! In each area, your old wise beekeepers will have strongly held theories about what works.