Today, information moves faster and is collected in greater volumes than ever before. Basically every project of meaningful size needs a data strategy--everything from websites to elections. If you're not collecting the right data, you can't make the right decisions.
This trend has made data one of the biggest competitive advantages you can have. Naturally, this means every business wants to hoard as much data as possible. At this point, it should be no surprise just how much data large companies have on your likes, interests, behaviors, routines, etc.
Privacy laws make it so you can request your data from any company and even ask them to delete it. This gives you certain control over your data, but it still doesn't belong to you. Despite the data being about you, it all belongs to the companies that have collected it.
Blockchain doesn't prevent people from collecting data about you; however, blockchain does create opportunities for data privacy. Every account on a blockchain is represented by a public key and secured with a private key. This simple design allows anyone to encrypt data, knowing that only the target person will be able to decrypt it.
If used properly, companies can collect, store, and even attribute data to you, but be unable to access the contents of that data without your approval. This is the level of data privacy that gives not just control, but full ownership of data back to the end user.
P.S. This is a constant pressure of centralizing data and power. Giving ownership over personal data back to users is just one step toward decentralization.