Any type of liquid, even water, will do fine. Although agar is best recommended as it has a lot of nutrients, allowing lots of bacteria to grow. Other liquids work but they don't grow as much and as vigorously.
They grow bacteria in agar because it is a controlled environment, a great medium for testing. Just to list a few reasons: -Test for the mobility of bacteria. Agar can be made to be more 'soft' or...