You may have nothing to hide now but what if your (political) opponents reach a point where they have access to your data and the (political) power to use it? What happens if they don't like your opinions which (you think) you don't have to hide now?
My opinions may mostly align with the current general consensus in my country and since I'm not politically active I am rather unlikely to be harmed because of my opinions in the foreseeable future (unless I call someone 1 Pimmel). But there are certain developments that are troubling and there are people who don't like what I've said on the internet (duh). Now, I'm not exactly anyone important and realistically there are far more important targets than me personally. But still, it's not unthinkable that the things I've said (things I've looked at on the internet, things I've bought, things I've like/upvoted) might be used to my detriment if certain people came into a position where they have access to any stored data on me.
This applies regardless of your political leanings. If data exists, no matter how harmless it may seem, there's always the possibility of people who REALLY don't like it getting access.