One of the most effortless way for an intruder to gain access to a system is "Password Guessing". As we can read from the NIST Special Publication 800-118:
Guessing attacks can be mitigated rather easily by using a combination of two methods. First, ensure that
passwords are sufficiently complex so that attackers cannot readily guess them. It is particularly important
to change all default OS and application passwords; lists of default accounts and passwords are widely
available to attackers.
For the Raspbian (official Raspberry PI OS), the default credentials (username: pi, password: raspberry) are the same since the launch of the first device in February of 2012 so if you plan to use a Raspberry PI for any of your projects, the first thing to do is change those credentials. For increased security is even better if you completely remove or disable the default pi user.