Why is Liver broth (covered with a layer of 2% agar ) commonly used as a growth medium, when analysing cans to check for sterility or cause of spoilage?

Why is Liver broth (covered with a layer of 2% agar ) commonly used as a growth medium, when analysing cans to check for sterility or cause of spoilage?.

Why is Liver broth (covered with a layer of 2% agar ) commonly used as a growth medium, when analysing cans to check for sterility or cause of spoilage?