The IMEI identifies mobile equipment on cellular networks, while a serial number is usually a manufacturer identifier used for production, service and warranty workflows. Many phones have both.
A serial number can help support teams confirm purchase and repair history. An IMEI is more useful when the question involves device identity, cellular capability, model family or network-related checks.
When buying or servicing a phone, compare both identifiers with the box, invoice and on-device settings. Mismatched identifiers are a reason to slow down and verify the device more carefully.