The most important feature is the ability for a developer to check their code/configuration against the Quality Clouds instance by doing a live-check. So the developer does not need to wait for a scan to confirm no issues were introduced, this can be done directly
Quality Clouds helps us to control our defined coding standard. The frequent scans will highlight potential problems and even categorize them in criticality levels. This increases the control over the developments and platform, not at least when we need to upgrade to a new major ServiceNow version. The code ad hoc testing capabilities gives you the confidence to stick to defined guidelines and best practices, right at the moment you actually develop code without waiting for a scan. Certainly, extremely useful!