This is a great question, central to the game. With chapters 1-5, we are met with challenges to build our city, and it seems as if we are laying down a foundation that we can continue to build upon, and mostly this is true. With the guest race chapters this philosophy changes a bit. There are still things we can continue to build upon, but there are also these temporary things that only last through the chapter. It can be confusing, or it can be an opportunity to have some fun with new things. Fun is generally my choice, but sometimes it does make me ask the question, why do I need to build this stuff?
My answer to the original questioner may be too philosophical, but I think it is why I like this game.
Ask yourself, what do I want for my city? Why do I want to progress through the chapters? What is the soul of my city?
This game is about helping others, and designing something you enjoy inhabiting. There are many choices, not enough in my mind, but certainly quite a lot. I don't think it is about reaching the ultimate goal, beating the final boss, gaining the last reward chest, etc. IMO
Finally, to my knowledge, the Dwarf stuff is just useless paperweights once you are done with the chapter. Sell it all. This is generally the case with the guest race chapters.