Headphones can be your best friend or your biggest hinderance to providing a great sound. Headphones are used so you can isolate the sound you want to hear and tweak that sound via volume or eq settings accordingly. Seems like it would be perfect for using for an entire service, right?
Lets step back to the sound check for a worship team. Let's say a drummer, two guitarists, a pianist, and two singers. A fairly easy group to mix. Listening to them practice, you notice that one of the guitar's needs some eq tweaking. You can tweak in the live environment but for starters, you don't know which guitar it is.
Using headphones, you can pipe the mix directly into your ears and thus tune out people talking or other stuff going on in the sanctuary that is making noise. But how do you isolate the guitar so you know which it is, without bothering the worship team? Each channel on the mixer has a PFL button. By pressing the PFL button, you isolate teh sound to just that channel – or channels if multiple PFL's are pressed. Now you can use the PFL to identify the specific guitar and tweak the EQ as needed.
Additionally, the PFL is great without headphones when you have one instrument or vocalist that blends with the others but it much louder. The PFL will modify the mixer's volume display so only the PFL'd channel is reporting it's volume. This is great for quickly finding the loud one.
Headphones can also be used to mix overall sound. Where I find they are deficient is through setting the overall sound for the sanctuary. The volume level via the headphones can be perfect but when the headphones are removed, the volume is too low. They also don't take room acoustics into account.
By using headphones for channel isolation, you can find and fix the mix for the best sound. However, if you leave them on, you might be the only person hearing that sound.