Comment by Arnt

One past employer of a friend has an internal network using IPv4 only. Every night a database query runs on one database and updates a second database based on the results (a DSS updated from a data warehouse, I think). One of the TCP connections involved goes through five levels of NAT, internally to the company.

No one on the team liked adding the fifth NAT, but no one felt confident enough to undo any of the old NATs either.

If you use IPv6 internally you don't dig yourself into holes like that one. You have enough addresses that you can choose clarity and maintainability in little day-to-day choices and a few years later that clarity has added up.