You can find a list of servicing stack updates at Latest servicing stack updates.
Starting in November 2018 new servicing stack updates will be classified as 'Security' with a severity rating of 'Critical.'
In rare occasions a servicing stack update may need to be released on demand to address an issue impacting systems installing the monthly security update.
Servicing stack update are released depending on new issues or vulnerabilities. If you don't install the latest servicing stack update, there's a risk that your device can't be updated with the latest Microsoft security fixes. Servicing stack updates improve the reliability of the update process to mitigate potential issues while installing the latest quality updates and feature updates. Why should servicing stack updates be installed and kept up to date? The CBS is a small component that typically does not have updates released every month.
Additionally, it contains the 'component-based servicing stack' (CBS), which is a key underlying component for several elements of Windows deployment, such as DISM, SFC, changing Windows features or roles, and repairing components. Servicing stack updates provide fixes to the servicing stack, the component that installs Windows updates.
