The main reason blockchains are decentralized is to avoid putting control in the hands of a few, or a country’s central bank. That’s the main motivation behind the embrace of cryptocurrency in the first place: to take banks out of the equation and have true peer-to-peer transactions. As was the dream of satoshi
Decentralized blockchains are designed to be unalterable, and once the data is entered it is irreversible. New data can be tacked on, but the old data can’t be edited or changed in any way. For Bitcoin, this means transactions are permanently recorded and viewable by anyone. Think of it as feedback on eBay taken to the next level.
Not all digital currency is decentralized. There are also cryptocurrencies that use private, centralized systems, where only a select few people have the power to add new blocks and check the validity of transactions. These tend to be used in privacy-oriented industries like healthcare and finance.