As a reader, I couldn't care less for an explanation of magic. What I
do care about, is consistency. It's easy to see when an author hasn't set rules for the magic in his/her world. I'm left thinking, "How did
that happen, and why didn't he do it before?" Magic without a cost; magic with no bounds; magic used to fill plot holes. All these are abhorrent.
The author needs to write down the rules for magic in his world. I have, and I abide by them. But why should I discuss them in a novel? Seems like a waste of words, when most fantasy fans won't care.
And discuss where magic comes from, or why it exists? I know both of these answers for my universe - but how many characters in my world would know this? (In fact, the number is 3. Three of the four arch-mages in my world know these things.) So no, I'm not about to discuss these things in a novel, either. (In a story I wrote long ago - published in a fanzine - one of those arch-mages alluded to the "sources" of magic, but did not reveal them.)
The main reason I care less for sci-fi than fantasy is those authors often find it necessary to give scientific explanations for things in their universes. Many of those explanations are pages long. Frankly, I really don't care
why those islands float in the air: That the phenomenon exists is awe-inspiring! It should be left so, without the explanations that take away that awe. Likewise, I don't want t know the inner workings of a phaser, or why a silicone-based lifeform might exist.
Enough ranting.
Just another two-cents from your resident bearded Fantasy-Freak!