You want to simulate a realistic load test where every user is having a unique user session. You application may only accept one user login per account at a time. You want each concurrent user to pick a different login password.
It's way more realistic to simulate concurrent users using unique logins and passwords because it's closer to the real world load. It's a good practice to avoid using a single user login and password for all concurrent users. Typically, web servers create web sessions per account. Using a single account for multiple users can mess up things on server side pretty quickly.
JMeter provides a facility to manage login and password: CSV Dataset Config. By defining a CSV file containing all the login and password couples, JMeter can pick a line from the file on each user iteration and assign them into variables.