Many synonyms have slightly different meanings or connotations. For example, "pretty," "beautiful," and "handsome" all describe physical attractiveness, but "handsome" is more often used to describe men, while "pretty and "beautiful" are more likely to be used to describe women.

Usually, the context tells you which one to use (like Susan's example of "handsome" for men and "beautiful" for women). This is just part of learning English. If you really don't know which one to use, just guess and ask for feedback. Native speakers will tell you which one sounds most natural. :)

Interchangeably