The importance of a good fret leveling job cannot be understated.
Frets are domed by standard, so when you play a string the point of contact on a fret is at the top of the dome and in the middle of the fret. However when you get a fret job you are flattening this dome and making the the top of the fret flat. Thus when you press the string now the point of contact is now often at the back of the fret and not in the middle, thus changing the intonation slightly.
A good luthier will reshape frets one by one after a fret level so that the frets are domed again, thus correcting the intonation.