If you have a light bulb and you want to be able to switch it on and off, then you must connect the switch and the light bulb in series across the battery.
If you connect the switch and light bulb in parallel across the battery, then the light bulb will shine all the time, and PLUS ... when you flip the switch to ' ON ', sparks will shoot out of the switch, the wires will get hot and smoking, and the battery will instantly empty itself.
So this is an application where the series circuit might work better than the parallel one.
Another example could be Christmas tree lights. You have sometimes hundreds of lights wired in series. If they were in parallel the wire would then have to go to each light meaning potentially hundreds of different wires
A data flow cannot go directly back to the same process it leaves. There must be at least one other process that handle(s) the data flow, produce(s) some other data flow, and return(s) the original data flow to the beginning process.
