I've just assumed that it tends to imply 'technology' via the long-existing term "I/O" [input/output].
Even the businesses that that use it and are less tech-heavy in business model tend to be web- and/or mobile-first type companies from what I've seen...
In addition to the "techiness" of io, a lot of the draw has been availability. They've been going at $50/yr for a while, which has kept the squatters away. Last time I looked, there were a plethora of three-letter English words available.
I jumped in in 2009, and basically had my pick of 2-letter domains.
Selling the remaining list is quite a smart idea. it obviously has value even though it would be relatively simple to figure out the remaining free domains on your own.
True, the task is not very difficult if you have a list of words to check against. If you're time to create your own list takes more than $19 of your time then its probably of some value to just buy mine :)
Even the businesses that that use it and are less tech-heavy in business model tend to be web- and/or mobile-first type companies from what I've seen...