This is a Wehrmacht ability. Though this might be one of the least flashy abilities found in the game, it should not be underestimated by any means. It is a passive ability that doesn't cost you any resources, and thus you don't have to remember to activate it at the right moment. Flanks from incoming enemy squads will be spotted much earlier, which helps you react to enemy movements properly and reposition your units. This is especially useful for your MG42s - you can adjust their cone of fire to counter your opponent's flanking maneuver.
Unless your strategy is based around an extremely early bunker, and you want to unlock Fortify the Perimeter at exactly 2 CPs, it is often smarter to choose this as your first ability. When playing with Defensive Doctrine, this gives you a nice boost without limiting the use of your normal Munitions-based upgrades or abilities. In contrast, For the Fatherland forces a trade-off between using that ability or an purchasing an early flamer, MP40 package or two mines.