It has a few properties that make it excellent for antibiotic use. First of all, it is a non-selective, non-differential medium. This means that almost all organisms plated on here will grow. Additionally, it contains starch. Starch is known to absorb toxins released from bacteria, so that they cannot interfere with the antibiotics. Second, it is a loose agar. This allows for better diffusion of the antibiotics than most other plates. A better diffusion leads to a truer zone of inhibition.