Orks are just silly. In the grim and/or dark future of the 40th millennium, Orks just want to beat the snot out of the enemy because they can. And they can because they believe.

Which is why they're wonderful.

Orkz fight because we gut at it

WAAAAAAAAAAAAAAAAAAAAAAAAGH

Orks do bring some humor to the game, but they're also terrifying, brutal and badass at the same time. That's what I like about them the most I think. The fact that they can bring this occasional humor while maintaining the grim dark setting is what makes them great. From an outside perspective, what they do can seem silly or even funny, but up close you see these grim antics usually end with the deaths and maiming of entire worlds. Grimdark at its finest.

Also this.

We'se the boss! WE CRUSH 'UMIES! WE'RE BEST COS WE AVE BIG GUNS ND CHOPPAS! GET THIS WAAAAAGH GOING!!!! WAAAAAAAAAAAGHHHHHHH!

We'z da onlee boyz dat can 'ave a gud tiome. Da udda gitz be too busy not diein' ta ave sum proppa fun.

Also, human slaves are treated better than normal citizens in the Imperium. Join the WAAAGH!

We all know dat green iz best! Now get back ta work, we've only got 2 yearz ta get ready fer DA WAAAGH!

Look at da Humies: Da tincan boys say deys be fighting for da Waaaghboss called Emprah, but da Emprah is mucking about and never joins da fight. Dey are followin' a git, and dat makes dem gits.

Look at da Spiky Humies: Dey also say dey be fighting for deir gods, but we got Gork an' Mork, and deir brutal cunning and cunnin' brutality beats dem chaos gods any time. Like dat one time dey punched da smelly one.

Look at da pansies. "Ooooh, we iz da doomed peoples". "Oooh, I broke off me fingernails". Da Eldar are so whiny, even Grotz are meaner dan 'em. And dat means somefing.

Now look at da Orkz. Da Orkz are made fer fightin' and winnin', and deys are green, so dey'z best anyways. Da Orks don't fight for some lazy git dat is to bizy sittin' on a loo to show up, and dey do not fight for some puny gods dat get punched in da face by Gork an' Mork, and dey are not whiny 'cause fighting is what dey to best, and dey sure love it.

Da otha gitz think dey could win da galaxy with some cunning plans or somefing, but da Orkz have already won and still are winnin all the time 'cause dey love fightin' and dey get fightin'.