You don't need to know number of candidates if you know the total amount of time (and assume candidates come in uniformly at random).
The most practical use case is selling a house.
There are also variations of the problem which model being able to go back to previously rejected candidates, as well as a probability that the candidate rejects you.
Right, selling a property might actually be a fairly good application of this model, I agree - you shouldn't leave a property listed for too long, and together with the feedback from your agent you should get a meaningful estimate of how many offers to expect in that period.
I'd be curious to see the agent's reaction when someone actually implements that and flat out rejects all the first offers.
The most practical use case is selling a house.
There are also variations of the problem which model being able to go back to previously rejected candidates, as well as a probability that the candidate rejects you.