Hervey says:

Allan,
Your question is not clear. I will try to interpret what I think you are asking. I believe you are asking why to put an insulated rail joiner and where or when to do so. The reason for using insulated rail joiners are varied.
Foe turnouts they are used between a live frog and the adjacent track. The reason for this is the frog changes polarity depending which position the turnout is in. There does not have to be a break in the bus wire but the frog is powered through a switch that will ensure the frog and any rail on the routed side are the correct polarity for the train to move smoothly through the turnout.

Another place that insulated joiners are used is in a reversing loop or a reversing Y. In both these cases you are turning a train 180 degrees so that the train is returning in the direction it arrived from. Powering of these mainline sections adjacent to loop or Y so the train can flow through without stalling needs changes in polarity With DC you this is usually done by routing power through a double pole double throw switch so you need to isolate the loop or Y from those adjacent sections. With DCC you usually use an auto reverser to control the polarity of the track in the loop or Y so that any short caused by the train when entering or leaving the loop or Y is so short that the train doesn’t stop. There is usually no need to have a break in your bus but the feeder wires are run through the appropriate control mechanism.

Finally you will use insulated rail joiners to separate blocks. In DCC you do this to control your power consumption and reduce the the impact of a short to the block in which it occurred.The block will have its own bus that will not be connected to other blocks but usually run through a circuit breaker. In DC you usually run a power supply to feed the block and control polarity via its own bus.
Hope this helps and answers your question.