Oh sorry I didn’t tackle this part.
Basically you defocus when you want a wider beam. That can be handy for making wider score lines (they get a little hazy around the edges if you defocus a lot), or if you want to smooth out the texture of an engrave. This works really well on acrylic, check it out:
The tradeoff is that you lose some sharpness. There are ways to mitigate that with a quick edge score, but it’ll be up to you to decide when to use the technique.
As for when it makes sense: it’s up to you! This sort of thing is a “feel” kind of situation. I’ve said it elsewhere before, engraving is as complicated a thing as we can do with the GF, and it’s where artistry plays the biggest hand. You can make relatively small tweaks to settings and get really different results for the same piece. It all comes down to what you’re trying to do and your preferences.