For the same reason it’s important to use someone’s name: respect. If your name is Brianna and someone decides that actually, no, it’s Amanda because you look like an Amanda and proceeds only to call you Amanda, it would be deeply disrespectful. The same thing applies to people’s pronouns. If someone has told you what their pronouns are, using those (and not ones you think would “fit better”) allows us to understand and respect that person.