Best it is explained with an example:
I am in New York, booked on flight LH401 from New York - Frankfurt. I would like to get notified if the inbound flight has a delay or not.
In my case it would be LH400 but as a regular user I wouldn’t know that info.
I could query of course flights from Frankfurt - New York and then calculate the inbound. But there are routs flown so much that it is not easy to figure out what’s the inbound flight.
Best would be to get the registration/tail-sign and query that.