...others have labored and you have entered into their labor.
This both keeps us humble and encourages perseverance. Often when an itinerant preacher sees a harvest during a meeting we fail to realize he is picking another man's fruit. A faithful pastor planted that seed year's ago; a broken-hearted mother kept it safe through her loud cries and tears. So this verse helps us keep perspective if we might be on the end of that process.
But we are also encouraged to persevere. Each word, sentence, and thought spoken is another log in place. Every prayer offered and tear shed tightens the bundle a little more. And we never know but that the next laborer may bring with him the coveted fire from Heaven.