Some customers find that they need to maintain two separate SIEM solutions to get the most value for each purpose, as SIEMs can be very noisy and resource intensive: they typically prefer one for data security and one for compliance.
In addition to the primary use case of SIEM for logging and log management, organizations use their SIEMs for other purposes. Another use case is to help demonstrate compliance with regulations such as HIPAA, PCI, SOX, and GDPR.
SIEM tools also aggregate data that can be used for capacity management projects. You can track bandwidth and data growth over time for planning growth and budgeting purposes. In the capacity planning world, data is key, and understanding your current usage and trends over time allows you to manage growth and avoid large capital expenditures as a reactionary measure rather than a preventative one.