For some reason Mandarin thinks a degree qualifier is needed even when you just normally use adjectives. So, for the meaning to be same, Mandarin usually can't say structures like English
You are beautiful.
Which does not have a word to qualify the adjective's degree. 
Mandarin usually uses words / constructions like
满/蛮...的 to express when there is not really any remarkable degree qualification.
好, it is used to mark subjectivity: that the quality is a subjective feeling or evaluation, which varies from being just a subjective marker to meaning "very", much like how the word
很 is in Mandarin.
The counterpart to subjective would be objective: that you are merely stating a fact. English makes this kind of distinction with word pairs like "very" and "so": "You are so beautiful" means roughly the same as "You are very beautiful", except that the speaker is signaling a subjective opinion.
You could say your sentence with the objective counterpart: 你很美 would be stating something like "you are beautiful as a matter of fact". Sure, it could ultimately still boil down to the speaker's subjective opinions if we talk philosophy, but the meaning being conveyed is different.
 (What is the nuance of 很好? Explains when the degree qualifier can be omitted.
Basically, when you are stating some quality absolutely, as if attaching such a label -- which implies a comparison like the one described in the accepted answer.)