microSD is a very common form factor, and cards are used in a vast array of consumer and professional devices. Because they are removable and user-replaceable, microSD cards give the owner of a device the potential to keep it useful for longer by increasing its storage capacity, or even swapping out cards to use for different purposes.
The ability to augment the capabilities of a device in this way means that the microSD card business is big. Unfortunately, because of the size of the market, there are many fake microSD cards for sale. Fake microSD cards might deliver less capacity than they claim, or they might not work at all. Worse still, they might stop working after a short time, or corrupt some data, so that what you’ve stored on them is no longer available. Fake cards also undermine the legitimate market, costing trusted manufacturers money they could put into improving their products.