I have a couple of these.
They are in places where my full sized fire extinguishers won't fit.
Like the tiny cabinet above my stove, and next to my shop door.
I have another fire extinguisher in the kitchen, and another 2 in my shop.
But I figure the more at hand, the closer they are when you need one. (Kinda like a firearm.)
They do have expiration dates, and they can't be that great as they are so small, but they are better than nothing.
A LOT better then throwing a cup of water on a grease fire.
(Don't do that! Use something to smother the flames!)