You shouldn't pull more than ~1 to 2A from a motherboard fan header which means you can probably only have 1-2 fans per fan header. Also controlling the motherboard fan headers from the OS is always finicky (most BIOS are utter garbage anyway). This USB adapter pulls power directly from the PSU so it can have 5+ fans and can be more easily controlled from the OS. This specific one can also control some LED strips, etc.

TBH it's not really necessary but people like to have it.