Smoke Grenade Pocket is a gear perk that lets you carry extra smoke grenades. Smoke breaks line of sight, so more of them means more escapes and more chances to reposition unseen.
You hold more smoke grenades at once. Throw them to cover a retreat, a revive or a dash across open ground.
Wear the Photojournalist Vest in the chest slot. Gear perks are fixed to the piece of clothing that carries them, so you get this one by wearing that piece, not by unlocking it in a menu.
Useful if smoke is part of your routine. Most players use other throwables more, so it is a situational chest pick.