I use some of these chips, and took a random stab at one of the files to see what it contained, a SPI-bus i/o expander. It looks pretty useful.

I usually write my own drivers, but that gets to be more and more effort if I want to make use of more sophisticated functions, plus I can make mistakes by mis-reading the datasheet or just counting wrong. I might be discouraged from using a function that I don't quite understand, but a pre-written driver might get me past that.

Note that these "drivers" are routines that talk to fairly low level hardware ports of a Linux computer, and I might modify the code for use on a microcontroller without an OS.