Sample standard deviation used to approximate the population standard deviation. I can imagine this was very handy when you were dealing with a large population in pre-computer era.
Now thanks to computers one can calculate the population standard deviation quite easily. I am wondering is the population standard deviation still useful? Why?
You could calculate the population standard deviation if you had data on the whole population. But in many cases of practical interest you don't. All you have to work with is a sample. Computers have not changed that fact.