Reynolds number is the ratio of inertia force to viscous force. It tells us about the type of flow. When the Reynolds number is higher than the critical Reynolds number for a given type of geometry it is regarded as a turbulence flow and when it is below the critical Reynolds number it is regarded as a laminar flow.
Depending on the type of flow further calculation of friction factors and other heat transfer-related quantities such as heat transfer coefficient are calculated. Hence whenever analyzing any thermo-fluid flow system it is a must to calculate the Reynolds number.