You are better of doing a search on this subject
For me its a way to monitor the tank if something dies and you don't see it , or if you are using ozone to raise ORP
ORP is used in the sewage indusrtry. It is also used to monitor ozone production which the sewage industry uses.
I have not found much use for it nor have I found any good science to support its use in reef aquarium. It is fun to watch ORP values and attempt to look for correlations to what is happening in your aquarium. I had one years ago and collected reams of data. Fun, but little value to animal husbandry.