In the Indomitus era, Primaris Space Marines are king amongst the armies of the Imperium. Primaris Marines form the bulk, if not the entirety of most chapters' battle and veteran companies, whilst the Firstborn, recently graduated from the 10th company's scouts are relegated to the reserve companies until they've proven themselves worthy of crossing the Rubicon divide.
Amongst the ranks of the Ultramarines, the Firstborn of the reserve companies spend most of their time performing such vital tasks as standing guard in the various hallways and thresholds of the unassailable Fortress of Hera on Macragge, practicing's parade drill for the tourists of Ultramar, and if they're particularly lucky, garrisoning some imperial backwater, freeing up the Primaris marines to do the real work of smiting the Imperium's foes.
Recently, the Ultramarines'' 7th reserve company was rewarded for their particularly efficient cleansing of the roadside trash along the Roboute Guilliman (no longer) Memorial Expessway's entire length. Thus they were honored with the privilege of guarding the porta-johns along the parade route leading to the Imperial palace on the forthcoming Day of the Emperor's Ascension celebration on Holy Terra.
As the 7th company prepared itself for it's sacred duty, thru what can only be explained as a clerical error, Lt. Wild, a combat squad from one of the 7th's tactical squads, and a razorback transport and it's crew were requisitioned to detach from the rest of the company, sully their newly polished armor by repainting it in urban camouflage of all things, and report to the staging grounds of the 3rd Battle Company's Strikeforce Horridus where they were to be attached in support of an unspecified mission.
Outraged at these various indignities, Lt. Wild and his accompanying marines nonetheless followed their orders to the letter and presented themselves to the 3rd's staging ground in time for the boarding of their transports. Puzzled at the Firstborn's arrival, and in matching camo no less, the 3rd's Primaris Chaplain paused in his exhortations of 'In the Emperor's name, GET YOUR ASSES MOVING!' to inquire: 'Are you little guys lost?'
On the rare occasions in which 'The Wild Boys' have been engaged in combat, they operate more in conjunction to, rather than in support of their Primaris brethren. Indeed the only support that they've directly provided on the battlefield is the transport of Primaris casualties* back to the transports via their Razorback.
*For those who insist that Primaris marines cannot fit into a Razorback, I would like to remind you that those would be 'whole' Primaris marines, whereas casualties tend to not be...