Creeps are auto-created inside your team's Fortress. Groups of four to five spawn at the beginning of the game and then continue to spawn every 15 seconds afterwards. The Creep spawn time can be upgraded to reduce time between each respawn cycle with an ability point. Their primary function is to capture Outposts and they will automatically head towards the nearest one. Like all other Infantry, once all Outposts are captured by your team, Creeps will head towards the enemy Fortress to attack it directly. Every 5 minutes the creeps will be upgraded (up to level 4).
Using a Bot Spawner, Creeps will be produce easier, their timer for spawn is about 13 seconds (10 to 4 seconds in late game) and level up after a minute has been passed, using this socket unit will allow the creeps to level up easier in less than 2 minutes. Best way to produce creeps is to produce it on an Outpost with several sockets, adding both Offense and Defense at the same time
Creeps are your run-of-the-mill infantry units, and should be used to capture Outposts. When used together with Bot Spawners, you'll have huge amounts of free Infantry running around on the map at any time - a great aid to map control. However, if you have low level Creeps (this will happen early game), it is essential to "door drop" them, meaning to be fast and drop Creeps right at the entrance of Outposts so as to immediately capture them. Also, some maps like Vale and Simple have certain Outposts heavily fortified by neutral forces. Thus, to maximize the potential of Creeps instead of letting them run to an Outpost just to die from heavy tank fire, you first have to take the units guarding the target Outpost out. Make sure to bunker extra creeps in the bunkers at the doors of each outpost, because if one creep dies in the outpost it can be replaced immediately by a creep in a bunker, effectively doubling the outpost defense. Since they spawn by default, there really isn't any dominant matching AirMech.