I'm using the 256GB Samsung card that a coworker got as a bonus when he purchased a note 7. He insisted on giving it to me even though I repeatedly pointed out that he could use it in two of his cameras and that he might get a note 8 himself. But he's got his heart set on an iPhone X, and said he had plenty of storage for his cameras.
He also wouldn't take any money for it. But he'll be getting a lot of free lunches for a while, I can tell you That!
Anyhow, I use the external storage mainly for music and photo storage, as well as downloaded PDFs, etc., which would otherwise be sorely limited by the measly 64GB that the phone came with. Some apps will likely also get installed there or moved there, too.
I also have almost all of the cloud turned off because I don't trust it. I backup to PC and external HDDs regularly, but maybe that's silly. I guess we'll see.
It's just nice to have the extra space, mainly for music and photos.