Since yours were only guesses, I offer these as considerations: i. A person cannot identify with someone else's sin by water baptism. But Jesus did that on the cross. ii. A person cannot be water baptized as a validation of a preacher's ministry, John said that he was unworthy to officiate over Jesus baptism, not that he was unworthy to carry out the ministry that God had called John to do. iii. A person cannot be water baptized to mark the official beginning of their public ministry. An ordination does that. iv. The pharisees and sadducees were there getting water baptized by John. v. Our water baptism illustrates our death and resurrection which could not be an illustration until after Jesus had died and risen again. Jesus said that his being water baptized was to fulfill righteousness. Those who upheld the law in it's strictest form where there to be baptized by John, clearly it would have been a violation of Jewish law not to undergo baptism by the divinely ordained baptizer, John.