I write addons because I am lazy and hate repetitive tasks.
I'd much rather spend time doing something interesting (writing and testing my code in a game) than something boring (updating a spreadsheet, checking my bag space before going into a dungeon etc).
That said, I also like the fact that my work can help others as well.
I enjoy helping others within the author community also.
To date, I think writing addons for games has got me the following that can be measured:
* Premium membership of another website for free (and therefore several beta keys & discounts).
* Approx £150 of Amazon vouchers.
* A $20 paypal payment (a guy in warcraft asked me to write him an addon, I did so for free. He then sent me $20 without me asking as all his guild used it loads).
I've never asked for money or donations though.