RHEL 10 must require the change of at least four character classes when passwords are changed.
Use of a complex password helps to increase the time and resources required to compromise the password.
Password complexity, or strength, is a measure of the effectiveness of a password in resisting attempts at guessing and brute-force attacks.
Password complexity is one factor of several that determines how long it takes to crack a password. The more complex a password is, the greater the number of possible combinations that must be tested before the password is compromised.
How this relates to the STIG checklist: STIG vs SCAP benchmark.