In the previous verse (11), Benjamin introduced the idea that he had been made king so that he could serve the people. He expands that concept. He has performed his service and contrasts an implicit expectation for what kings do. It would have been expected in most cultures that the king would enrich himself. It was sufficiently common throughout the world that the king would be considered the wealthiest, and that much of the labor of the people would be sent to the king to enrich him. Benjamin declares that he has not done that.
The phrase “neither have I suffered that ye should be confined in dungeons” begins a series of things that Benjamin has not done. The only reason for stating what he has not done is to contrast the implicit understanding that these are things that the people would know that kings do. There is no reason to suggest that he hadn’t thrown people in dungeons if that never happened. There is no reason to declare that they should not make slaves of one another unless that was a possibility that was known and understood.
His conclusion is that in contrast to expectations, he has even “labored with mine own hands that I might serve you.” Whatever he had done, it allowed him to decrease the potential tax burden. Thus, Benjamin could show that he, himself, had labored for their benefit, rather than the expectation that the people labored to support a king.