The ability to fold is single handedly the greatest evolution in mobile phones for some time. Apart from the folding form factor, phones have been stagnating for some time with only SoC upgrades and camera sensor upgrades.
Surely a gimmick is something not useful, being able to expand your usable screen estate from a pocketable form factor can't be described as not useful? It's very useful for content consumption and web browsing, and to some extent is great for productivity. I say some extent because there are genuine use cases for split screen (I've lost count the amount of times I run the calculator app alongside banking apps for example), but I still don't find the bigger screen useful for content for making Excel/Google sheet documents, mainly because those apps are watered down and/or have too many UI elements event for a tablet screen.