on Writers Helping Writers:

Side writing: Any exploratory piece of writing that helps a writer get to know elements of their story but isn’t intended to make its way into a draft in its entirety. Examples include journaling from a character’s perspective, writing a scene from an alternate point of view, or creating sensory word lists for a particular setting.
Writers tend to feel strongly about side writing, one way or the other. I’ll admit it. I was not a fan of it for most of my writing life. Sometimes, I still have trouble working up the power to sit down and write, period.
But I’m going to make the case that whether you love it or despise it, you owe it to yourself, your story, and your reader to write one piece of side writing. Just one: the origin scene.