Luke 12:37. "Blessed are those servants whom the master finds awake when he comes; truly I say to you, he will gird himself and have them sit at table, and he will come and serve them." 

Later Jesus will ask: "[W]hen the Son of man comes, will he find faith on earth?" (Luke 18:8). Today's verse from Luke tells us how much He will reward the ones who He does find faithful upon His return - He will gird Himself and serve them! However, in another parable, Jesus talks about a servant coming in from the fields and He asks whether the servant's master would tell him to "Come at once and sit down at table", or would he instead tell the servant to prepare supper for and serve the master? (Luke 17:7-10). In other words, we are doing no more than is required of us when we continue to serve God, and we should not expect reward for it.

So why does Jesus flip the narrative here and say He will serve the ones who remain faithful until His return? I think it is because the fields in which the servants will be working will become tremendously treacherous and difficult in the time before His return, and many of His servants will abandon their tasks out of despair, exhaustion, fear, or temptation. Jesus tells us that it will become very difficult for us when we follow Him, especially in the times before His return. So while the servants who do their duty in the fields, but then are called home, are given no more reward than being able to continue to serve the Master in His home (which is a tremendous reward in and of itself!), the servants who remain in the fields as the battle comes to them, and remain faithful, are deserving of a bit more.

The servants who are in the fields at the time of His return and have remained faithful are the ones who will see the Church through to the end. They will not be called back to the Master's house until the battle is done; they will remain fighting and faithful through the worst of times. Jesus will reward them in a special way for that, just like a general returning to his undefeated, but greatly reduced and battle-weary, army would praise them for their faithful service.