Ask three teams for a list of systems and you get three different answers. IT has a spreadsheet, security has a slightly older one, and the privacy team keeps its record of processing somewhere else entirely. None of them agree on what counts as critical, who owns it, or what data runs through it. Every audit starts with a week of reconciling lists that should have been one list all along.
The inventory is where everything else starts. Classification, risk scoring, business impact analysis, your GDPR record of processing, the register of AI systems: all of it reads from the same set of assets. Keep four copies and they drift apart by the next quarter. Keep one and a change to an asset shows up everywhere it matters, the first time you make it.