I have dental insurance through my employer and for routine dentistry it works like this:
1. Visit dentist
2. Dentist advises I need a filling and carries out the work
3. I pay for the treatment
4. Submit claim to insurer and they pay.
There has never been any dialogue about pre-existing conditions. My policy even covers routine check ups and cleaning.
There are exclusions e.g. they don't pay for any antibiotics prescribed by the dentist. One time my dentist said I needed a tooth removing but I woud need to be in hospital with a general anaesthetic. Then I had to go through the routine with the insurer of geting them to approve the surgeon, the anaesthetist, etc. before having the op.
I have found it very worthwhile particularly where some work has been expensive - crowns and the like.