For example, the 50uAh EnerChip CBC3150 is rechargeable 5000 times at a 10% depth of discharge to 80% of rated capacity.
Pretty bold to call it 50uAH if only 5uAH is usable without trashing the battery.
And how marketing-wankery of them to use "at a 10% depth of discharge" in the hopes of readers believing they meant "to a 10% state of charge".
At least the datasheet says "Charge/discharge cycles: > 5000 times at 10% discharge", which is clearer and less wankery. And lower down, they do admit that at 40°C, only 500 charge/discharge cycles at 50% discharge.
The only use case I can think of these is when you have a bare die on your board under a blob of epoxy, and want to hide a small battery for RTC/RAM backup there to keep some Super-Secret Sauce
TM like the most important one, License/Warranty Expiration Date. You know, to "optimize" device life.