This living organism can be converted into different buildings.It can attack air or ground by different modifications.The advantage of this production type is,the reduced instantaneous cost.You give the some part of the mineral and after that you can give more mineral to convert this building for different purposes.The choice is yours.Thee creeps colonies has another ability,they make the flow of the Zerg creep by their secretion systems.You can expans your creep area by these buildings accourding to your tactic.But here you must be careful incorrect creep location means extra mineral lost.