1. Raine and Julia show the theme of love persevering. Even though Laguna and Julia could never be, Julia's child and his, Laguna's child, did in fact meet, fall in love and live happily together.
2. Raine's introduction shows us Ellone who, as Spoony did rightly say, is the axis on which the entire plot turns.
3. Laguna's importance is to contrast with Squall. He was the hero before Squall was. Squall is only doing so many of those things because of Laguna and the choices Laguna made.
4. No, Seifer's Dream doesn't stand on its own because NO GOAL stands on its own without explanation. You'd have to be a smurfing awful storyteller to think something for something's sake is preferable to explanation and reason.
This is a waste of time. The Laguna flashbacks each serve a purpose storywise and I shall illustrate it now.
First Flashback: The scene with Julia shows two main character's parents. It shows the romantic and lover Laguna is.
Second Flashback: Shows how Laguna is torn away from his happy prospects.
Thrid Flashback: Shows how Laguna has come to love another woman and introduces a central plot figure.
Fourth Flashback: Shows the origin of Seifer's motivation.
Fifth Flashback: Shows how Esthar was freed from the tyranny of Adel.
The purpose of any and all flashbacks is to establish a past plot point that helps us better understand how things currently are in the narrative. Guess what all the Laguna flashbacks do?
Oh and
why was Squall in the orphanage in the first place? You guessed it - showed in teh flashbacks.