The ancient greeks when listening to Homer, would be quite familiar with the story already, having heard the stories over and over since they were children, and very few of them probably ever experienced the story from "the beginning', but rather heard snatches of it here and there from story tellers, and in fact, the story is optimized for _retelling_, rather than _telling_, and presumes the audience is familiar with the characters and events.
I'm fairly familiar with the Greek gods, but the problem I've had with reading the Illiad is that it is not at all obvious why this is written. With The Odyssey it's clear: he's trying to get home. But everyone in the Illiad seems to do stupid things for unclear reasons and I just loose interest. I think I would be helped out a lot by finding some summary of the themes the poem is talking about. So I think it's good advice.
Similarly, I'm currently reading through the Poetic Edda. I thought I had a decent grasp of Norse mythology, but I am clearly missing references left and right. (I know, because they are footnoted) I think reading the Prose Edda with its background first would probably have been helpful.
You're right. Still, if anyone were to ask for my opinion I'd tell them that you only ever get one "first read", and that might as well be the book itself. If you can't make heads or tails of it (in this case that'll be rare), then sure, look at some secondary sources.
Even if you don't struggle, the secondary sources are great to read through after the fact. Stuff that'll help you get even more out of it on a second reading.
Thinking abo