Well uncompressed raws from one of my cameras are 47 MB each, and this is not a particularly high-resolution camera in 2017; add to that a second camera with smaller raws but still significant size, and mr_pingu said, do some bracketting, timielapses or videos, and you'll easily fill a 64 GB card in just a few hours. The µSD on my Jolla is the same size as the SD in the cameras, so it can really help when one is full while you're out taking pictures.

It happened to me several times on trips with no access to large storage or computer, had to find ways to use my phone as external storage using someone else's computer or cybers, and even bought horribly slow (and expensive) USB sticks in my last trip because the phone was not enough.

Admittedly, I could just buy new SD cards (I already have spare ones but they just are not enough with such big files during long trips), and I will, but then there's what you should do and what you do.