Recently it came to light that hackers stole 3 billion login credentials from Yahoo. It had been previously known that Yahoo had been hacked, but this is the first time that the number of hacked accounts has been acknowledged to be so high. The information included names, email addresses, birthdates, telephone numbers and passwords.
This information is valuable to hackers because it can be used to hack into other accounts. For starters many people reuse their passwords on different accounts, so the hackers may be able to use the email address and password and see if they can log into banking accounts. The additional information they gain may also allow them to access your other accounts using your birth date and other information they may have found to convince the password reset or help desk to reset the password.
If you have a Yahoo account, you need to change your password on any accounts that have the same credentials as Yahoo. In future, you may want to use different passwords.
Another danger in this attack is that email accounts have much of the information about our lives. If the attackers downloaded the emails of selected users, they could then mine the emails for other accounts to attack for information necessary to assume those selected people's identities.
If this data was stolen by a state actor or sophisticated syndicate, then this data could be combined with other online information to get a better understanding of people they are targeting. Especially if they managed to get the emails of the people that they are looking to target.
Online information should not be considered private. Assume that all online information can be compromised and avoid adding information that could be embarrassing or compromising online. Also, try to use different passwords for different accounts. If that is too difficult, try to use different accounts for different types of information.