If you are writing a series, a family tree is great to at least know (even if it is only written out and not developed as an actual family tree). It helps with consistency / continuity between books in series. You can do it for your character's nuclear and extended families, if you like. You can add it to a series bible, as one of the elements.
Author and Youtuber, Brittany Wang, has a basic series bible template you can consider for things to add to continuity....
https://www.authorbrittanywang.com/writingtips
Her series bible doesn't add much if you are writing science fiction (such as alien species, worlds and moons, vehicles and their weapons, melee weapons, short-range weapons, long-range weapons, and many other entries).
If your series becomes popular, fans will call you out if your details do not match up. I heard of fans getting vicious for stuff like this comparing Harry Potter and Fantastic Beasts.
You can take the family tree to another level and create a "character relationship chart," but that's a whole, other level of complication. Google search what I added in quotes and click on the "Images" link at the top. You can see some schemes people tried to make these work. Too much work for me, but I can see it being useful.
Character backstory is great to draw upon conflict during the story. For example, a long, lost distant relative may come into the story and affect the MC's character arc. You may need to draw upon MC memories of this relation and what opinion he/she has of them. Family members are great ways of adding conflict to stories of MC transformation.