The primary function of a double block and bleed valve is for isolation and the secondary function is for intervention (diverting / sampling / injection etc).
Double block and bleed valves operate on the principle that isolation can be achieved from both the upstream and downstream process flow / pressures. This is achieved by two ball, gate, globe, needle, etc. valves placed back to back, with a third "isolatable" valve in the centre cavity. Once isolation has been achieved in one or more of the main process isolation valves, the cavity that is created between these isolations can be drained. This is useful for flow diverting, sampling or injection situations, and for maintenance / integrity check situations where seat leakage can be monitored through the third "bleed" valve.
Double Block & Bleed Valves - General Design Criteria
Multiple Configurations with Ball, Gate, Globe, Needle Options