Linked from
The 12 pages that link to Mass flow rate, each with the reason it gives.
Thermodynamic systemRelated: It quantifies matter crossing the boundary of an open system.
Mass balanceRelated: Inlet and outlet flow rates provide the balance's transport terms.
CompressorRelated: Compressor capacity is commonly specified by the mass of gas delivered per time.
Specific impulseRelated: It supplies the propellant-flow term used to calculate specific impulse.
Jet engineRelated: Thrust depends on how much air and exhaust the engine moves each second.
Material balanceRelated: Flow rates quantify material entering and leaving an open system.