A faceclaim can be a key piece of a character. Its how they can be described first meeting another character, it can be how an RPer is better able to envision the scene in their head, and so much more.
In some RPs, you merely see an FC once in the slides with the rest of their information, while in others it can be beneficial to have a known FC with many pictures for a social media slide. In that case an FC becomes associated that much more with a character, and is part of them just as much as the key parts of their background.
So when you see the FC of a certain character, in a situation not relating to that character or RP, do you think of that character?