The Multiple Meter Streams options can be used to more efficiently compute flows through a single meter at a physical site that has different fluids flowing through the meter at different times. Since each Stream will have it's own meter configuration, do it's own flow calculation, and store it's unique accumulations, it's almost like having up to four flow computers built into one.
For example, a refinery might ship various products through a single distribution point (meter) but never more than one product at a time (such as batches injected into a pipeline or loaded into tanker trucks.) In this situation, they must choose whether to measure the different products using different meters or attempt to use the same meter with for all of the products. Since meters (especially turbines) are 'expensive' and so are flow computers, it is financially advantageous for the refinery to use a single meter, if at all possible. Then a flow computer with multi-stream capability can allow the one meter to act like four.
So, the primary benefit of using multiple streams is a more efficient use of the flow computer. On a single stream flow computer, It is possible to assign different products to separate flow computer meter channels, even if only one physical meter is used, by doubling up all the process inputs into each of the flow computer meter channels. However, in the case of turbine meters with single-stream flow computers, each of the doubled-up channels would receive all the pulses from the meter, even those channels not being used to measured that product. This skews the accounting -- unless complex compensatory logic were implemented in the PLC to feed pulses only to relevant meter channel. With multiple-stream support, one can assign multiple streams to a single flow computer meter channel (in the case of the AFC up to 4 streams), thereby freeing other meter channels for assignment to other meters.