Valek is a dumb demon or the screenwriters are
Why would the demon give them his name? That makes no sense.
shareWhy would saying it back to him make him go away? That makes no sense either.
shareIt's an exorcistic given that knowing a demon's name gives the exorcist power over the demon. Letting a demon know that you know its name, then throwing the name at the demon will at least partially depotentiate him...supposedly.
shareYes, but why give them his name? If you were a demon, would you give them your name so they could have control over you?
shareNo, you don't have power over the demon just by knowing it's name. That's superstitious hogwash.
The reason you ask a spirit's name is simple: you want to know what you're dealing with. Sometimes, it's more than one spirit/demon causing the grief, and they're afflicting the person (or house, or object) for different reasons. Maybe a demon is haunting a person due to a cursed object, or maybe the person committed a heinous crime that the demon has latched onto.
It's like going to the doctor, and the doctor asking you a series of questions to determine what's causing the issue. Knowing who the demon is and why it's harrassing a person or object can get them out quicker.