Defensively, it's the best Toxic Spikes setter.
Eternatus is the next culprit, and no, it is not primarily because of having to fit a Kyogre check, although that is definitely a nice trait. Rather, it is because of the sheer prominence of Toxic Spikes. On one hand, Eternatus is the most viable Toxic Spikes setter and beats every relevant Defogger, but on the other hand, it is also the only viable Poison-type capable of removing them. I don't think it is hard to see why this leads to awkwardly linear and one-dimensional intricacies in the teambuilder. While there are definitely a few outs versus Toxic Spikes, such as utilizing Gothitelle to trap and get rid of Eternatus, or simply minimizing the amount of Toxic Spikes-prone Pokemon on your team, using an Eternatus yourself has definitely been the most practical solution to Toxic Spikes in my experience.
So, we can tell that Eternatus is very viable in Galar AG, mainly due to checking Scarf Kyogre and the best toxic Spikes setter in the meta, and being able to beat most defoggers thanks to its bulk.
It isn't bad at offensively too.
Eternatus is very difficult for a lot of balance and stall teams to deal with, owing to its great offensive presence paired with its colossal bulk, Toxic immunity, ability in Pressure, and access to reliable recovery. Dynamax Cannon is a powerful STAB move that 2HKOes most Pokemon that don't resist it and hits Dynamaxed Pokemon twice as hard. Flamethrower targets Steel-types such as Zacian-C, Necrozma-DM, and Ferrothorn that resist Eternatus's main STAB move. Sludge Bomb OHKOes Xerneas, preventing it from setting up on Eternatus, and has a significant chance to poison foes. Alternatively, Meteor Beam allows Eternatus to punish Ho-Oh seeking to pivot into it, while also making Eternatus progressively harder to deal with due to the Special Attack boost. Recover lets Eternatus offset Life Orb recoil and take advantage of most Pokemon that can't 2HKO it, including walls like Ho-Oh, Ferrothorn. It's also useful in tandem with Pressure to quickly drain the PP of opposing Pokemon.
Eternatus fits well on balance teams that appreciate the offensive and defensive utility it provides.
Apart from that, Choice Scarf is also a good set. While it may sound weird, it can help against Dynamaxed Yveltal since it can outspeed it after a +1 Max Airstream. It can also have Shadow Ball for Calyrex-Shadow, but without a doubt, Yveltal does it better since some are Focus Sash to irritate you.
Overall, Eternatus is best used defensively as the main set of "T-Killer" or Toxic Spikes Killer. Hope this helps!