demographics info, connections between things that people search for (people who look for 3D also look for Maya also look for Subsurface Scattering also look for Titty Physics) for search engine optimization, how popular franchises and new films and games are.
things other than straight up ads use this information, corporations will make decisions on what to fund and market outside of the internet with information datamined from it, and websites like amazon will take the datamined information to better supply suggestions for additional purchases.
also 4chan is just one piece of a larger web of information to gather and users on 4chan can be traced to other websites that may also use the same services for gathering data. there are many ways of tracking individual users beyond their IP addresses, such as the versions of software installed and their installed fonts. with enough variables you can get a strong fingerprint. https://panopticlick.eff.org
i don't have my computer fully noided so while it does block a lot of things, it still has a browser fingerprint that appears to be unique because firefox reports all of my addons and screensize and OS and the versions of my addons etc.