In Puerto Rico, what is the significance of coins being put each round on people that are not chosen?
It has two purposes:


As you say, it ensures that every role gets taken, and that no role goes entirely unloved for too long. There are multiple paths to victory in Puerto Rico, and most of them require some combination of roles to be taken: a shipping strategy requires Settler/Mayor to set up and Craftsman/Captain to execute, a building strategy requires Builder plus Craftsman/Trader (or some other income methods), etc. Putting coins on unloved roles means that if 3 players go for one strategy and one player goes for a different one, that one player's favorite roles (or unloved-but-required supporting roles) will still be chosen, even by people not following that player's strategy. It pushes all of the players at the table to make the game friendly to all strategies.
Some roles are weaker than others, and putting money on the roles makes them more tempting. Craftsman is a classic "weak" role, because the person who takes it will usually have last choice in the next shipping/trading phase. (Unless they're clever with the end of the round and the movement of the Governor role.) This is mostly just fine-tuning the power of the various role cards: if some cards are weaker than others, then add a little cash so they're not quite so weak.