Spoils the twist the Cloud is impersonating Zack if you play this before playing the OG or the remakes. Which is very important to the story. Its like watching the prequels before watching empire strikes back. And if you play it after, the only context it adds isn't entirely canon. Sephiroth didn't learn his motivations from Genesis, Genesis wasn't in the mako reactor during the nibelniem incident in the remakes or the og game.It gives more story context to Sephiroth, Zack, and Aerith. If you care about the story and went straight from Remake to Rebirth without playing this and Intergrade you might not appreciate those characters story arcs as much.
Probably more for people who haven't played or don't remember the original but I'd probably still be wondering what motivates Sephiroth if I didn't play this