Nobody is being asked to give a specific endorsement of any specific candidate. They are all being asked, in general, if they will support the nominee whomever that is.
If someone asks "why would anyone do that", it's because it benefits all of them if the eventual nominee has as much support as possible because they all want to be that nominee and have as much support as possible.
Whether each of the candidates is willing to make that promise is up to them. I just think that trying to maximize support for the eventual nominee is squarely within her job duties.