They most certainly do not stay on the SD card. I dump the cards to my laptop immediately and to the backup drive connected to my home desktop as soon as I get there. SD cards fail more frequently than hard drives, although you have to plan for HD failures, too.

As for uses, I share some with friends and family via e-mail and Facebook, sell a few as a freelance and have some of the best printed and framed on my walls. I have given a few framed prints as gifts. I had an electronic frame on my desk at work before I retired and rotated those after each major photo trip to keep it fresh.