It is used when your vCenter is down so you can still retrieve passwords from SDDC manager. As SDDC manager uses vCenter as its authentication.
You can create or enabled the admin@local on sddc manager
Recommendation is to Save the FQDN/IP address of your SDDC manager, vcf, root, admin@local credentials so they are easily accessible
Below is the official VMware documentation link to enable or create the account, You can use Developer Center > API Explorer within SDDC manager to do this.