It lets you set up on stuff like ferrothorn easier than with substitute, and is also good alongside dragon pulse so you can eventually win vs non SD Bisharp and tyranitar locked into crunch.
In other words, (quoting from the analysis)
"Reflect Type enables Latias to freely set up on Pokemon that would normally force it out, such as Ferrothorn, Mega Metagross, Tyranitar, and most Fairy-types. Substitute is an option over Reflect Type, as it lets Mega Latias avoid status conditions, such as burn, paralysis, and poison, and play mind games with Sucker Punch users."
Ninja'd lol