Stories are arguably the most visible social media content available.
How so? Take your phone out and open FB or Instagram. What's at the top of the feed every time? Stories! I hear so many businesses lamenting that they don't have the social media visibility they desire, and then I notice they aren't even consistently using the stories feed to share their own posts, photos, videos, and more. It's hard to overstate the missed opportunity here.
It comes down to mechanics. Do you want to be consistently at the top of the feed on FB and Instagram? Publish stories regularly. And yes, you can share content from other accounts into your stories, so the pressure is off you coming up with content every day.
Anything goes with stories - can be your content, content from other sources, helpful info, articles, photos, videos, a mishmash of all, including photos and videos you may shoot in the moment and share to stories.
Note that they do expire in 24 hours (unless you click the heart at the bottom and "favorite" it which allows you to retain it in a collection on your profile). This actually makes them more attractive to the consuming public. They like the idea of seeing current, relevant, and unique content that may not be around tomorrow.
Also note that you can make a story on one channel and share it as the story on the other. Example: make a story on Instagram and click FB on the publish screen (or just turn it on permanently) and your content will flow to the top of both feeds.