From a purely selfish standpoint, beyond the issue of the public good, can this bring extra traffic, give you additional credibility, make you any money?
CC Attribution is a well-recognized way to quickly give permission to people to redistribute and remix your work, publicizing it and increasing its value in the process. Bloggers looking for more color for their posts might grab a relevant CC'd photo.<p>CC Attribution-Share Alike may lead to someone creating something that you can reintegrate with your original work, improving it. If you provide a dataset of 2D time-based points and someone makes a neat visualization of that, everyone is better for it.<p>This depends on your content. Things like photos or datasets will probably be reused and remixed more than, say, written content (which will probably be quoted and linked to regardless of license).
It answers the question of "how can I re-use the content of this site legally"<p>The upside is that it means the site creator has put some thought into it, and allows the user to understand their rights clearly.