Agreeing on truth with lots of involved parties that act on eye level is hard. Hierarchies and a central point of authority help here. Blockchain technology explicitly wants to avoid this authority figure and therefore had to solve the 'finding truth problem' which is also called 'permissionless or trustless consensus'. There are several ways of achieving consensus and different blockchain projects are trying them out. Each consensus model comes with risks and benefits. For Bitcoin it is 'proof of work' which makes sure the ledger (who owns how much and transferred value to whom) is kept intact and can't be manipulated.