Many applications need user accounts' functionality. Data associated with an account should be modifiable by the account owner only. DApps simply cannot have the same username- and password-based account functionality as do centralized applications because passwords cannot prove that the data change for an account has been requested by the owner.
There are quite a few ways to implement user accounts in DApps. But the most popular way is using a public-private key pair to represent an account. The hash of the public key is the unique identifier of the account. To make a change to the account's data, the user needs to sign the change using his/her private key. We need to assume that users will store their private keys safely. If users lose their private keys, then they lose access to their account forever.