Every CMDB starts the same way: someone runs Discovery, a few thousand CIs show up, and for about a month the data is genuinely useful. Then people start manually editing relationships to make dashboards look right, ownership fields go stale the moment someone changes teams, and eighteen months later nobody trusts the CMDB enough to use it for impact analysis. This isn't a data quality problem. It's a modeling problem.
The Common Service Data Model (CSDM) exists because ServiceNow's default CMDB structure lets you load in almost anything without forcing you to answer the one question that actually matters: what does this organization mean by 'a service'? Without that answer, you end up with a database of servers and applications that technically exist, but can't tell you what breaks if one of them goes down.
CSDM fixes this by giving you a layered structure (business capabilities, business applications, application services, technical services, and the infrastructure underneath) with defined relationship types between each layer. It's not a magic fix for messy data. It's a constraint that stops the messiness from compounding, because every new CI has to fit somewhere specific instead of floating around loosely connected to whatever seemed reasonable at 4pm on a Friday.
The part most implementations skip is the governance layer. CSDM gives you the model, but someone still has to own the decision of what counts as a 'business application' versus an 'application service' in your specific environment, and enforce it when new CIs get created. Skip that step and you'll have a technically CSDM-aligned CMDB that's just as inconsistent as the one you started with. The labels changed, the discipline didn't.
If your CMDB has quietly become something people stopped trusting, the fix usually isn't a re-discovery. It's going back to the handful of services that actually matter for incident and change (the ones finance or ops would notice in minutes if they went down) and modeling those correctly first, end to end. Depth on a few real services beats shallow coverage across all of them, and it gives you something concrete to point to when someone asks whether the CMDB rebuild was worth it.