The most obvious problem at the camera end of photography is memory card failure - SD card, CF card etc.
If you shoot commercial photography then choosing a camera system that duplicates your RAW images onto two cards is essential, your reputation, and by extension the success of your business is going to be largely based on your ability to reliably deliver images and “My memory card died”
is never going to be an acceptable excuse. You can be fairly sure of losing the client along with having to pay for someone else to re-shoot the work, so not having a photography backup strategy is career suicide.
However, I would argue that dual card slots are also essential for amateur photographers too. I’ve watched non-commercial photographers pour their heart and soul into planning and executing shoots, often hiring studios
, models and HMUA’s to create stellar images for their portfolios. Risking all of that time, money and effort on the reliability of a single memory card is crazy!