A character's motivation, what they actually want in a given scene and why they want it, is the underlying force that makes their subsequent choices and actions feel earned and consistent, rather than arbitrary and merely convenient for the plot. Experienced fiction writers routinely work out a character's specific motivation before deciding exactly what that character will do in a scene, because an action taken without a clear underlying want tends to read to a reader as unmotivated, something the plot needed rather than something the character actually chose.
An action without a clear want reads as the plot pushing the character, not the character choosing
When a character does something in a story purely because the plot requires it at that point, without a clear, established want driving that specific choice, readers tend to sense the disconnect even if they can't immediately name it, since the character's behaviour stops feeling like an expression of who that character actually is and starts feeling like a mechanism serving the author's external plot needs instead. This is precisely the failure mode experienced writers work to avoid by establishing a character's underlying motivation clearly enough, early enough, that later actions can be read as genuine expressions of that established want rather than as unexplained plot necessities.
Clear motivation makes later choices legible even when they surprise the reader
Once a character's core motivation is genuinely well established, that character can take actions a reader might not have specifically predicted, and still have those actions feel earned and consistent, because the reader can trace the surprising choice back to the character's already-understood underlying want, even if the specific way that want manifested wasn't obvious in advance. This is exactly why strong character motivation gives a writer real narrative flexibility rather than constraining it: a clearly motivated character can act in genuinely unexpected ways and still feel entirely consistent, since the reader has already been given the underlying logic connecting motivation to action.
What we're still unsure about
That clear character motivation is widely regarded, among both writers and readers of fiction, as central to what makes a character's actions feel earned rather than arbitrary, is well established craft consensus. What's more genuinely a matter of ongoing stylistic disagreement is exactly how explicitly a character's motivation actually needs to be stated on the page versus left for a reader to infer from behaviour alone, since different literary traditions and individual writers take genuinely different approaches to how much motivation gets spelled out directly versus shown only through implication, without one universally agreed standard for how explicit is explicit enough.
This sits inside Character Development & Motivation, one of seven topics in Fiction, one of five domains in Literature, one of seventeen subjects the app can quiz you on.