Being bigger and louder than the already-noisy Ork vehicles, the Big Trakks are designed to carry much more armament over the standard Trukk and are frequently used as gun carriers instead of troop transports. In addition, for many Mekboyz without the resources or know-how to build full Battlewagons, Big Trakk's are the next best thing they can realistically afford. Like all Ork machinations there are a plethora of prevailing designs for the Big Trakk, and on top of that each Big Trakk is crafted uniquely from any other. Some Big Trakk designs are favored by different Mekboyz and Clans, from the Blood Axes who often mimic Imperial patterns to the "kustom-built" Bad Moon gun platforms. The most prominent users of Big Trakks are the Goffs and Deathskulls klans.
They are the Ork's primary mode of artillery and fire support and boy are Big Trakks utterly important for an Ork army. As Orks have piss-poor aim and most of their units and vehicles are assault orientated, the Big Trakk (along with the Mek Gunz) are your only form of long range artillery support and anti-tank firepower. Whilst it is not in the same quality as a Basilisk Artillery Gun, it compensates this for its ease in customization and just plain old versatility. Having trouble with zerg rushes? Just put on a Skorcha or two and load up some Burna Boyz to roast some gitz. Being peppered by those annoying Weeaboos with their Markerlights? Just load in a Supa-kannon and proceed to blow up the general area with Steel Rain. Your Ork Boyz being hammered by Leman Russes? Just add in a Zzap Gun and proceed to turn tanks into metal omelettes.