With this small adapter, you can use MicroSD cards (no matter what size) instead of the much more expensive MemorySticks in a PSVita 1xxx / 2xxx or PSTV.
However, a custom firmware must be installed on the device (firmware versions 3.60 or 3.65 with HENkaku Ensō).
A good installation guide for this can be found here: https://vita.hacks.guide/yamt.html
Since I'm sure the question will come up more often about which version this is and whether it is then not outdated:
It is v3.0 - and yes, I know that there is v6.0. However, they are still technically identical (there's no great tech in there).
So why are there different versions? Well, there are several manufacturers - and each assigns its own version number. Some fit better in one or the other console version (others are a bit more sluggish), but technically they are identical.