The Reverse World

After you have defeated Necrozma, when you return to the Altar of the Sunne or the Altar of the Moone depending on your game, you will find that there's still a wormhole remnant. This is actually a small wormhole that you actually have the ability to go through, but there are a few conditions. First it allows you to go through to Ultra Space and Ultra Megalopolis First, you need to have either Solgaleo or Lunala (or Necrozma's fused forms) in your party. Second, the wormhole only opens up at certain times.

Solgaleo Solgaleo Only opens up when the game is at night
Solgaleo Dusk Mane Necrozma
Lunala Lunala Only opens up when the game is at daytime
Solgaleo Dawn Wings Necrozma

The alternate world essentially inverts the time, so Day becomes Night and Night becomes day. This isn't a version exclusive feature, so if you trade Lunala over to Pokémon Ultra Sun, you'll be able to activate the wormhole during the day.

The Effects

The effects are largely marginal and superficial. The time is inverted so AM becomes PM and PM becomes AM. This will allow for you to do time-based evolutions, hunt for time-specific Pokémon and events at all times. The areas Altar of the Sunne & Altar of the Moone will become their counterparts, as will the Lake of the Sunne & Lake of the Moone. This essentially makes the overworld of Ultra Sun be like the overworld of Ultra Moon, and vice-versa

Despite this, this feature does not change the wild Pokémon you'll encounter, so if you're playing Ultra Sun you will still encounter Vulpix rather than Sandshrew, and Rockruff & Cosmoem will still evolve into the version specific evolutions.

#789 Cosmog

Location: Lake of the Sunne (Sun), Lake of the Moone (Moon)

Another feature is that this is the only way to get the Legendary Pokémon Ultra Beast, Cosmog. Cosmog is not found in the native world, but is in fact found in the Alternate World. When you have gone through the portal go to the Lake of the Sunne or Lake of the Moone from Ula'ula Meadow and reach the altar, with the cover legends still in your party. Cosmog will then appear. You don't need to battle it, it will join you automatically. It can evolve into Cosmoem and then into another Solgaleo in Ultra Sun or another Lunala in Ultra Moon. You can only get one Cosmog per game. Once captured, you will then get the Lunalium Z in Ultra Sun and Solganium Z in Ultra Moon

Level 5
Hold Item:
??? Nature.
Date of Receiving
Lake of the Sunne/Lake of the Moone. Caught at Lv. 5
#745 Lycanroc

Another reason to do this is Lycanroc Dusk Form. In Pokémon Ultra Sun, to evolve the Own Tempo Rockruff into Lycanroc, you just have to level it up to Level 25 or higher between 5pm and 5:59pm. However, in Ultra Moon this is a bit trickier as, due to the inversion of time, you'd need to be awake at between 5am and 5:59am.

Using the Reverse World is a good way to get around this, as it would then allow for you to switch to have the time by 5pm and 5:59pm while in the Reverse World.