What does Matthew 20:7 mean?

Jesus' story of the master of a household hiring workers for his vineyard (Matthew 20:1) takes a surprising turn. Who hires temporary workers when there is only an hour remaining in the workday? Apparently, the work that needs doing in the vineyard, likely the harvest, is so urgent that the master requires all the help he can possibly get. For his own reasons, and under his own counsel, the master chooses to bring on these last-minute helpers.

The master has hired workers four times, at 6 a.m., 9 a.m., noon, and 3 p.m. The day is almost over, but he returns to the marketplace once more. He asks some available workers why they have spent the entire day just hanging around the marketplace. Now they answer simply that nobody hired them. They were ready and willing but not needed. The master needs them, though, and immediately sends them to join all the others working in his vineyard. No mention is made of what he will pay them. Perhaps they think that getting paid anything for a little work would be worth it at this point.