I mean with regards to availability. Whether they come jacked or not doesn't matter
You can enter the first section of the Underground as soon as you get to Eterna City. In the first section, you gain access to 10 unique hideaways: Rocky Cave, Spacious Cave, Grassland Cave, Fountainspring Cave, Riverbank Cave, Volcanic Cave, Sandsear Cave, Still-Water Cavern, Big Bluff Cavern, Dazzling Cave.
Rocky Cave gets you early access to
Skorupi, and additionally gets you access
Lickitung,
Rhyhorn,
Gligar,
Swablu,
Absol, who you wouldn't be able to find otherwise.
Spacious Cave gets you access to
Magnemite,
Teddiursa,
Houndoom, and the aforementioned Lickitung, Rhyhorn, Gligar, Swablu, and Absol.
Grassland Cave gets you early access to
Roselia (for those who don't like happiness evos) and additionally gets you access to
Scyther,
Pinsir and the aforementioned Absol. Perhaps more notably than anything else, this hideaway also gives access to
Combee and
Cherubi without needing to mess with Honey Trees.
Fountainspring Cave gets you early access to
Gastrodon (approximately 10 levels underleveled at its earliest availability) and additionally gets you access to
Togepi,
Ralts, and the aforementioned Swablu.
Riverbank Cave gets you early access to
Croagunk, and
Stunky, and additionally gets you access to the aforementioned Lickitung, Scyther and Pinsir.
Volcanic Cave gets you access to
Magby as well as the aforementioned Houndoom and early Skorupi.
Sandsear Cave gets you access to the aforementioned Rhyhorn, Houndoom, Magby, and early Skorupi.
Still-Water Cavern gets you access to the aforementioned Lickitung, Scyther, Pinsir, and early Croagunk.
Big Bluff Cavern gets you access to the aforementioned Scyther, Pinsir, Gligar, Absol, and early Skorupi.
Dazzling Cave gets you early access to
Ghastly (before Gardenia rather than after), and additionally access to
Smoochum,
Elekid, and
Duskull as well as the aforementioned Togepi, Houndoom, and Ralts.
In total there are
24 unique pokemon that become available as soon as you hit Eterna City, a number of whom are immediately useful against Gardenia and Jupiter.
7 of those pokemon are available in the overworld, but only much later.
And that's just the Eterna City location. More pokemon become available later in the game, including some like Munchlax who can be very difficult to find otherwise. Not to mention the levels of all pokemon in the underground scale with your progress through the story, meaning you pick up certain mons after battles they might struggle with and have them immediately make themselves useful with very little babying (such as Rhyhorn, who struggles with Maylene and Wake but destroys Byron and Candice).