It is also a good way to help friends because usually you give a lot of value to others very quickly.
In the same way, I also helped a birdwatcher to download thousand of bird calls. My own rant: http://blog.databigbang.com/the-call-of-the-web-scraper/
(obviously I don't endorse scraping Craigslist because of TOS. Just confessing to a past infraction back when the NYC housing market was even more absurd than it is today)