Jesus uses an everyday scene His hearers knew well. No master would tell a servant just in from plowing, "Go and sit down to meat." Commentators explain the purpose in two ways. Barnes says the parable appears to have been spoken "with reference to the rewards which the disciples were expecting in the kingdom of the Messiah," to show that they would be provided for, but that reward "was not the 'first' thing." Gill links it to the mighty faith just described (Luke 17:6), saying it was given "to keep the disciples humble in the performance of such miraculous works." Either way, the question guards the apostles against thinking their service puts God in their debt.

Jesus answers His own question with another: "And will not rather say unto him, Make ready wherewith I may sup, and gird thyself, and serve me, till I have eaten and drunken; and afterward thou shalt eat and drink?" (Luke 17:8). He asks again, "Doth he thank that servant because he did the things that were commanded him? I trow not" (Luke 17:9). Then He applies it: "So likewise ye... say, We are unprofitable servants: we have done that which was our duty to do" (Luke 17:10).

The word for "servant" here is doulos, a bond-servant. "Feeding cattle" translates a verb that means shepherding, which is why the ESV says "keeping sheep." "By and by" is old English for "immediately"; the Greek is eutheos, at once. "Sit down to meat" translates a word meaning to recline at table, the way meals were eaten (ESV: "recline at table"). In Luke 17:9, "I trow not" means I think not, or I suppose not, as Barnes explains.

The apostles asked for more faith (Luke 17:5). Jesus answered that even small, real faith can do great things (Luke 17:6), and then He told this story so that great faith would not turn into pride. Matthew Henry's Concise Commentary says that the Lord "cannot be in debt to them for their services, nor do they deserve any return from him." Matthew Henry also writes that "The best servants of Christ, even when they do the best services, must humbly acknowledge that they are unprofitable servants." Barnes sums up the servants' confession: "We have conferred no favor."

Yet this is not the whole picture of how the Lord treats His servants. The parable describes what servants may claim, not what the Master chooses to give. Earlier in Luke, Jesus said that the lord who finds his servants watching "shall gird himself, and make them to sit down to meat, and will come forth and serve them" (Luke 12:37). We cannot demand His reward, but He may still choose to give it.