It helps to keep in mind that Chinese "adjectives" are really a subclass of verb. Mei3 can mean not only "be beautiful" but "become beautiful". Also, there are no comparative or superlative endings in Chinese either, so mei3 can also mean "more beautiful". The way you say that someone is more beautiful than someone else is literally "Bob, compared to Rob, is beautiful" (i.e. "Bob bi3 Rob mei3").

Thus, if you simply say, "Bob mei3," the listener is left wondering, Are you saying that Bob is more beautiful than someone else? More beautiful now than he was before? Getting better-looking each day? AFAICT, using hen3 has the function of forcing a positive, stative interpretation. "I'm just saying Bob is beautiful without reference to how he was before or how he compares to anyone else."