The "render unto Caesar" line was put in by the King James scholars; it's not in the original Aramaic or Greek - and it was put in there simply because King James wanted to be able to collect taxes without so much fuss.
Never have I heard so much self supporting drivel for the rules of commerce as an example, or parable, to teach not rewards on earth, but rewards in following the lead of the church. The lesson hints, do what you're told in the name of god, and you'll be rewarded on earth.
The parable teaches, as tertiary history, do what you're told, and you're rewarded. It supports authoritarian lead-followship, by mixing apples and gonads--bribing the doer with riches as a fallout from doing what is told, using the attraction to money and riches to dupe goats to obeisence. The parable is meant to indicate being a good steward and doing what you're told by authority brings riches and rewards if you're obedient, and if so, you'll enjoy financial success, even that financial succss, is an indicator you're doing "it" right, namely selling your free will to the church.