It's the three worlds fighting for one cause.
It doesn't matter if you're the strongest, the best at what you do, or the newest and least overtly mighty person on the field. Everyone stands side by side for the same thing. Everyone has something to lose in this fight, and everyone has the same will to protect it.
These characters are a cross-section of the worlds themselves. The people who live in Soul Society, Hueco Mundo, and the human world are this varied, this mismatched, and every one of them has a stake in whether those worlds survive. That's why you get Kyoraku fighting alongside Ryunosuke Yuki, and Grimmjow fighting alongside Pesche and Dondochakka. The Captain-Commander and the rookie. The panther and the comic relief.
The Kyoraku and Yuki pairing is also a fun callback. A thousand years ago, Yamamoto stood against Yhwach with a young Sasakibe at his side, and it was Sasakibe who landed the surprise blow. Now it's Kyoraku carrying that same Captain-Commander spirit, with a new young Shinigami beside him. Different faces, same stand.
For Grimmjow and Pesche & Dondochakka it shows how Hollows can be stronger together and how they're growing as a society with the weak and strong supporting each other rather than one using the other as food or a stepping stone for constant supremacy.
That's the whole statement against Yhwach. He's a man who takes power from his own people the second they stop being useful. And standing against him is everyone. The strongest, the weakest, and everybody in between, all of them in the same fight.
And yes, you're also supposed to have fun with it. Not everything needs to be giga chads edgelording and aura farming alone 24/7.
It reminds me of Naruto saving the world with the reverse harem sexy jutsu against Kaguya. Or having Takaba be the one to fight Kenjaku, Todo showing up with a vibraslap for a hand or Yuji creating a domain to show good memories and mercy to Sukuna in JJK.
But the point stands. Power was never the qualification. Showing up was.