These boyz are a revamp of the old concept of Boarboyz as orkish cavalry. The big difference is that these Beast Snaggas ride on a vaguely horse-shaped squig simply called the Squig-Hog, instead of riding on warboars.
Precisely why they were invented in the meta is unknown, but presumably, it's to tie into the increased prominence of the orkish built-in ecology, and/or to give Feral Orks back their cavalry whilst distancing them from their Warhammer Fantasy (and now Warhammer: Age of Sigmar) counterparts. Their 'standard-issue' weapon is a spear called a Stikka - like their Imperial Rough Riders counterpart - except, instead of having an explosive tip, these Stikka have a rocket booster attached to its ass so when the Boy throws his shaft, its gonna fly and hit something from a distance with enough force to pierce power armour.
Other than that, they may also carry a Slugga as a sidearm or have a trusty Grot saddlegit armed with additional firepower like a Squig Blasta for example or with some ol'fisty cuffs for close encounters.
On the tabletop, these are your primary Ork cavalry units. They are pretty durable and quite strong as well, and between the Ork's stikka and the squig's own jaws each model is a serious threat to MEQ equivalents, monsters, and vehicles alike. Even the saddlegit is able to provide some extra melee and dakka punch.
The Ork's stikka (which, as previously mentioned, is actually a rocket-assisted spear) can also be used as a ranged weapon that becomes even more deadly at 6" or less. The whole mob can also be accompanied by a Bomb Squig or two, which is a really nice way to inflict some mortal wounds in those clutch moments.