Same goes as in real life, the appearance of a person kinda influences the first impression of said person
On the internet, that profile picture is your face.
For me personally, I sometimes make some basic assumptions on the person based on their avatars, like what possible fandom they're in, what their interests are.
Of course, it is not the most determining factor when judging a person, you wont be able to know one's full personality and story from a simple picture which is their pfp.
In conclusion, profile pictures or avatars act the same as your face in real life. They give someone a first impression about yourself before interacting further with you. But of course, the wise saying of not judging a book from its cover still applies when interacting with people on the internet.