It matters if 1) you have long-lived sessions on the application (RP) or 2) the application (RP) does something on behalf of the users and you need to stop that.
For example, say I leave the company. My account in SSO (IdP) gets deactivated, so I can no longer use SSO to log in to the company GitLab. However, without some way to let GitLab know that I'm gone, I might still be able to access repos with my SSH keys or access tokens, and scheduled jobs in my account will still run. Deprovisioning not only lets GitLab know I won't be logging in through SSO again, but also that it should stop the scheduled jobs in my account, and block other access methods.
You're right that depending on what your application does, you might not need it at all.
For example, say I leave the company. My account in SSO (IdP) gets deactivated, so I can no longer use SSO to log in to the company GitLab. However, without some way to let GitLab know that I'm gone, I might still be able to access repos with my SSH keys or access tokens, and scheduled jobs in my account will still run. Deprovisioning not only lets GitLab know I won't be logging in through SSO again, but also that it should stop the scheduled jobs in my account, and block other access methods.
You're right that depending on what your application does, you might not need it at all.