The Tragic Love Song of Destiny

It seems the intention was to provide more characterization for Kikyou and how Inuyasha's relationship with her came about. In particular, it pays a lot more attention to the idea that Kikyou felt trapped by her role as a priestess and wanted to be a normal woman, and hoped that by using the Jewel to make Inuyasha human, it would free her from her responsibility to guard and purify the damn thing.

That said, it's mostly filler, invented for the anime. As with other filler episodes and arcs, it was a means by which the anime held off catching up to the manga for a little while longer.

"You always know where the X-Men have been, because it's always on fire."
- Pete Wisdom

I love it, myself. Besides the points already mentioned, I also liked the origin of the spider explained(as something of a ringleader of the Naraku demons), and how the timing of even more events was tied together from the same period.

i liked it because kikyo is so evil in the anime and movies (heard shes sweeter in the manga) and the special made her more sympathetic. it just makes people understand why inuyasha still has feelings for her even if he loves kagome in the present, because of the bond shown between him and kikyo. and i dont know why manga fans take such offense that its not canon when, as you said, its almost the same as shown in episode 15 and the manga chapter. i guess its the diehard inuyasha/kagome fans who dont like the fact it made some people not hate kikyo and kikyo/inuyasha anymore

The scene where Naraku appears as a spider as he witnesses his buddies being slayed without mercy adds depth to his character.