This is a compact Unix joke.

The structure
    case
    ...
    esac

is the most economical way of delimiting the case statements.
it saves having to invent some extra blah, like:

    case
    ...
    end case


It is the same with the conditional statement

    if whatever
    then
      blah
    fi


You could find other things like this ...
e.g.

to display a file on the screen we use
   cat file

to display a file on the screen in reverse order (last line first)
   tac file