Single sign-on lets a person authenticate once, through a trusted identity provider, and then use several connected applications without signing in again. One account and one set of checks covers them all. For a team that uses a dozen tools each day, that saves a surprising amount of time spent typing, resetting and remembering passwords.
It reduces password fatigue and the habit of reusing passwords. It also helps the business: when someone leaves, switching off one account removes access everywhere, and extra checks such as MFA need setting up only once. The central login becomes important to protect, so it should be well secured.