Predatory edge is an upgrade (50 gold, 100 mana) that increases the foot speed of Crawlers and allows them to attack while moving, giving them an advantage over unarmored units (Swordwraths, Archidons, Miners, etc.) and prevents said units from successfully retreating.
The main purpose of Predatory Edge is allowing Crawlers to chase and maul any unit stupid enough to retreat. While this upgrade, when coupled with Pack Mentality, allows Crawlers to kill any unit with average speed and low armor, any unit with an armor above four or possessing speeds greater than a bomber can still successfuly slaughter or outrun the crawlers, respectively.
Unlike Pack Mentality, this upgrade is unaffected by the number of crawlers you have so even one crawler can successfully chase a fleeing unit. Still, it is recommended you purchase Pack Mentality while or before purchasing Pack Mentality since Crawlers deal inferior damage without it.
The game guide recommends using Crawlers with Pack Mentality to harass miners, for Miners are too slow (even with Hustle) to escape the initial attack and are doomed afterwards since the Crawlers can still attack while moving forward.
This upgrade also allows a swift retreat if things should go wrong.