To fully understand the implication of verse 9, we need to back up and read a few verses before it. Paul discusses how both he and his companion, Apollos, played a different role in planting seed (sharing the gospel) despite the rising tension of fractions within the church. Paul is quick to point out that it was not Paul that made the seed grow nor Apollos rather, it is God who makes things grow.
In verse 9, the language shifts from drawing upon an agricultural example to an architectural example which reminds us of the importance of a solid foundation holding fast to the ways of God. God could have chosen many ways for word and care to be shown throughout the world. Yet, He chooses us (humans) as His co-labors and workers.