By comparing preachers with labourers in a field, Paul assigns to them a much humbler position than that which the Corinthians have been assigning to them. Labourers had a low status, being either slaves or day labourers. Paul probably has in mind day labourers, since he had only come in to plant,
and Apollos only to water.
Moreover, their work would be useless if God did not give the growth. This confession is in line with Paul’s earlier emphasis on the work of the Spirit (1 Corinthians 2:14–16).
7 So neither he who plants nor he who waters is anything, but only God who gives the growth.