Fiction can serve many purposes. The primary purpose, I'd say, is simply the enjoyment of immersing yourself in a made up story. It's a peek into a world vastly different from our own, one in which events take place that couldn't possibly take place here, and characters with extraordinary traits. The enjoyment of reading the series comes from reveling in their faults and virtues, the fantastic plot, and the imagination of the author.

Fiction can also be used to teach lessons, of course. WoT is full of lessons, some of which are subtle and others not so much. Tar Valon is a beautiful representation of an entity with immense power steeped far in tradition--and how its failure to change with the times brings it weakness.

Rand's lesson is one of love for humanity. It's hard to argue with this one; it's also arguably the most important lesson of the series.

Nynaeve's lesson has been one that mirrors the lesson of Sai'dar--learning not to fight directly that which is too powerful. Overcoming her block of anger was something that went hand-in-hand with her development as a person; learning to actually listen. Ironically, she's grown more as a Wisdom by becoming Aes Sedai.

Egwene and Perrin both follow similar lessons in leading.

The Seanchan gave us an amazing insight into the importance of completely disparate viewpoints. I felt this was the most GRRM-like writing he did, showing (what we considered) complete bad-guys, owning slaves, and we somewhat understood their point of view. Doesn't come close to the 180 that readers turn with the likes of Jaime Lannister, but it's the closest we get. GRRM has very few totally evil personalities, but they are in there.

Overall, I got great joy in reading the series and also following the theories online. I'll be sad when it's over for sure, but we've all grown quite a bit older as the series as progressed, and it can't go on forever.