standard error context query

Hi, Probably a blindingly obvious answer to this - but please could someone clarify? My understanding is that the standard error is the standard deviation of the sample mean to the population mean. And required to calculate this is the population standard deviation. Why would you be taking samples if you already know the population standard deviation and presumably therefore the population mean? Is this to do with taking samples of a very high number, such that the sample standard deviation can be used as a proxy for the population standard deviation? Thanks for your help, Andrew