For certified companies, ISO 9001 requires in chapter 9.2, among other things, that internal audits are planned, carried out and documented.
These self-inspections are intended to ensure that an appropriate and functioning quality management system is in place.
They are a valuable means of repeatedly putting your own system to the test, uncovering gaps or weaknesses and improving processes. Internal audits should not be seen as an annoying ISO requirement to be fulfilled, but should be used as a real opportunity to produce better quality more efficiently.