Because you are not a unique and special snowflake. If you regularly go to /r/The_Donald, it says something specific about your politics (probably). Same for /r/LateStageCapitalism or /r/trees. It might not say much, but it adds up to a profile of who you are and what you think about.
If you are emailing certain people, or tweeting them or whatever, GCHQ can build a social graph of people you know, who they know, etc. If you are the friend (or friend of a friend) of a person of interest, you're more likely to be of interest yourself. There are not many criminals like the una-bomber working entirely on their own - most of us need encouragement and/or provocation, and nowadays much of that happens online.
If your search phrases include things like "how to make a bomb", you're probably going to be on a database somewhere. There have been numerous serious court cases (e.g. murder trials) where the prosecution have presented evidence that the accused's search history included phrases like "how to dispose of a body" or "how to poison someone". In other cases, jurors have been dismissed for using Google to research the background to the case they are serving on. I wonder where the information about these searches came from?
Metadata is important for identifying "interesting" people. When you have found them, you "zoom in" and start hoovering up all the information you can find, not just the metadata. It's the greatest spying tool ever, and a way to implement highly repressive government too - just start monitoring people with different lifestyles or "way out" opinions.
http://ghanadailies.com/2016/11/22/uk-government-plans-porn-...
https://www.theguardian.com/commentisfree/2016/nov/23/niche-...