A CMMS is the system of record for maintenance: every asset, every PM schedule, every work order and every completed job lives in one place instead of scattered across spreadsheets, paper logs and whoever happens to remember. That record is what makes MTTR, MTBF and OEE measurable at all — you can't trend a metric you never logged.
The line between a CMMS and an EAM is mostly scope: a CMMS is maintenance-focused (assets, work orders, PM), while EAM extends into the full asset lifecycle — procurement, depreciation, capital planning. Most facilities need the former; only larger multi-site operations typically need the latter.