I do it mostly because sometimes I would like Google to track what I'm doing in certain websites, mostly related to my work, that way I get relevant Ads when visiting some blogs or open source developers, If i get to use some open source I usually click on an ad of the website as a way of supporting them. I have actually found nice services or people clicking random Ads.
My private FF sessions reside in bitlockered thumb drives shielded by every script blocker available, I do it from some laptop trough TOR, on some proxies I have installed in lousy maintained cybercafes. I don't do anything illegal but I'm sometimes commissioned to look after sensible information related to political issues or infosec. I don't have to do all this shit when browsing Know your meme, it's a waste of time.