Shared universes may have become the norm for popular franchises, but the CSI crossover episodes were common in the shows before so many movies were doing it. The CSI franchise started in 2000 with CSI: Crime Scene Investigation, a show that later would also be known as CSI: Las Vegas. The story about a team of forensic scientists working to solve the strangest cases in Las Vegas was simple but interesting enough to create a thrilling show that would air for 15 years and see three spinoffs and a revival, CSI: Vegas.

https://screenrant.com/csi-every-crossover-episode-explained/