I like publishing things I learn as I learn them.
Why? If I didn't, I wouldn't remember them.
Really, why? Okay, I also spend a nontrivial amount of time cleaning up and making my notes palatable for a general public, because I believe knowledge should be free.
Money? I love writing, but I also love my family. Tasks that help me provide are going to get preferential treatment. Combining the two would mean the world.
Donations? Ads aren't ever going to be an option, because I want to maintain a distraction-free design. Also a great way to get kicked out of any ad network is to openly support your readers in blocking tracking cookies.
Amount? Even the smallest, tiniest, loneliest, little penny will go a long, long way. I make my coffee at home and load up a thermos, so I get a lot of coffee out of each dollar.