There are several reasons why a fret dress may be needed. Some players like a very low action, and usually this can only be achieved by first getting the frets as even as possible. But usually it is worn frets, intonation problems, or dead spots due to uneven fret height - this particular case is the latter. I start by adjusting the truss rod to get the neck as straight as possible.

Here's the long straight edge in the middle of the neck. The aim is to get minimal gap here and at both ends.

I was on a web forum yesterday and somebody kept writing "Wala!" After a while the penny dropped, so I wrote back, "Do you mean Voila?" He came straight back saying "No dude, it's a guitar!" (Voila - Viola? Geddit??)