'cause it's an important element in plotting. Like the family situation. A character unencumbered with family -- an orphan with no siblings, spouse, or children -- is free in some respects, lacking in assets in others, both of which are useful in plotting situations. The social station of the character determines a lot. Education. Property. Connections. Which on one hand give them the ability to tackle various problems as they arise in the plot, and on the other hand give them ability to tackle various problem as they arise in the plot, which is bad because you don't want them to handle them too easily, that's where your conflict arises, them against something more powerful than them -- but still plausible (a serf's child would be flattened by threats plausible for those higher in station). So either the threats have to be metered to the social station, or the social station has to be metered to the threats.
And it also imposes constraints. Even with a minor estate, the character has to deal with it -- can he even afford a steward, if it's that minor? And fantasies in which the only child of the king goes gallivanting about on adventures are fantasies in more than one sense. Aragorn gets away with it because he is effectively a king in exile and so not a king. Boromir, OTOH, is in explicit pursuit of his role as the Steward's son -- the constraints are exactly what is needed if that's what the story calls for. The king sends his only son out into the wilderness so he will survive if the city falls, and so he goes adventuring, but you can't really get away from those constraints.