Whatis milk used for?
Dairies milk cows and sell it to make cheese, or they can keep it and drink it. It's really good! But the sooner the milk from the cow is chilled, the better it will be. You can use the creamy stuffi from milk to make butter.
"whatis" displays the one line manual page description for its argument "apropos" searches all the manual pages titles and descriptions for the argument and will list the one line manual page description for all the matching entries it finds. Therefore "apropos whatis" will find only one matching man page and will report the same information as "whatis whatis" command.