People don't realize how much data is collected, how it's analyzed to determine things about you, and how it's given out to nearly anyone. Here are some concerning examples that hopefully speak for themselves.
Data from fitness app Strava was used to locate secret US military bases in Afghanistan and Iraq by some random guy on Twitter. He did this by pointing out people running in squares in the middle of the desert. Imagine what America's enemies could do with this information that this company will sell to anyone.
Ad company Xandr allows you to target audiences with labels like "Recently purchased a pregnancy test", "Has a large gambling debt", and "Has depression". Once again, this is freely available for anyone to purchase. These tracking companies find out things that are very personal to you and then sell that info to people who might not have your best interests in mind.
Last but not least. Governments and law enforcement can access this information at any time for any purpose. Do you really want the government and police agencies to have a database of people grouped by their religious and political beliefs or their sexual orientation?
Hopefully you can see why the information being collected and given out to anyone is concerning. As to how to avoid it, I'm not sure there is any way besides government regulation. Maybe someone else has some answers!