Hello, I have Balanis' Antenna Theory 3rd Edition. I read on page 31 at
the very last sentence that "Side lobe levels of -20 dB or smaller are
usually not desirable in most applications."

Why? On the same paragraph it states that "minor lobes represent
radiation in undesired directions, and they should be minimized." If a
side lobe is a minor lobe, then why is it undesirable to have side lobe
levels less than -20 dB? Thanks!