Single Row Table for Storing an Additional Unique Identifier
I used to work with a program that used a similar system to generate unique IDs for invoices. It did originate in a different database system, one without auto-incrementing IDs.

There's one difference with your system, and it's a difference I like a lot. The invoice table has a unique, auto-incrementing ID - and it has a separate, customer-facing (I'll assume) invoice number.

I went through a process of merging two structurally identical databases. The auto-incrementing ID values overlapped, so we had to reset one set of them as we went through the merge process. Our invoice table used the unique, auto-incrementing ID as the customer-facing invoice number. So, we functionally lost those invoice numbers in the system.

While it would have been potentially problematic to have two invoices with the same (customer-facing) invoice number, at least we would have had that option if the customer-facing value had been distinct from the primary key.