I'm gonna be fully honest, this team is mostly a revamp of an old SM team I had that I really liked that was unfortunately really weak to Kyurem-Black. Since Kyurem's a lot worse in ORAS, I felt it would be a good idea to adapt that SM team to ORAS. Latias is a really cool pokemon, that defensively checks a lot of mons that rip through other teams, and it's a really valuable wincon if positioned properly, but it's so weak to status and has moveset problems as is, so refresh is usually off the table. Couple that with a nasty pursuit weakness and it doesn't get used much despite it's talents. Because of this, most games it won't actually sweep, at most killing 1 or 2 pokemon, but it always has the ability to be one of the best defensive stopgaps in the tier and allow you to check mons while your other members force progress. When I was building this team for SM, I just threw on a Tapu Fini as my status absorber for the team and made sure I had a gameplan vs tspikes and was good, but I had to be a lot more conscious about handling status as well as pursuit, since Fini is not in ORAS.
Heatran has been on the rise in ORAS, and it's surprisingly how easily a mon with this many tools was forgotten. It's especially valuable on this team, since it is immune to not one but two forms of status, and it shuts down CM Clefable that can be really annoying for balance. It also appreciates how Latias shuts down SD Gliscor and provides a ground immunity, while it in return actively denies Weavile and spreads burns, which actualize the possibility of a Latias sweep.
*insert meme about Excadrill being on every ORAS team*. Latias gets worn down by rocks sometimes=rapid spinner=excadrill. Heatran has stealth rock & toxic=substitute excadrill.
I wanted a ground resist for offensive excadrill and a blanket check to a lot of attackers like M-Metagross and M-Tyranitar, and Tangrowth's ability to remove leftovers and just constantly be annoying is very much appreciated. It also chips ferrothorn for tons with HP Fire, which is valued since Ferro shuts down M-Latias when at high health.
At this point, I felt really weak to screens, and having Weavile to annoy Serperior and revenge kill Manaphy after some chip was really nice. It also has really good synergy with Heatran, since the switchins Weavile forces (Clefable, M-Scizor, Ferrothorn, etc.) are all Heatran food.
Keldeo isn't as relevant in the current metagame, but it still has its place. The team was basically complete without it, and I wanted an emergency switchin to the likes of Heatran, Weavile, and Volcarona, so Keldeo was added.