Giving a candidate the impression that there are many other (serious) candidates for a position has some clear benefits for an employer:
It gives the employer a better position when negotiating salary and other benefits.
It increases the pressure on the successful candidate to accept the job offer.
Therefore, if during the recruiting process I am given to understand that I am the only serious candidate for the job, this is likely to be true. While I would usually assume assume no bad intentions, I may make strategical decisions based on this information. Therefore it is valuable for me to know whether my assessment that there is no reason to bluff about this (leading to my disadvantage) is true. Note that I am asking for potential, general motivations, and I am well aware that I have to be the judge whether they apply to my particular situation.