What is the function of in this 为 sentence?
为 means "for", as most people know. In this case,  "for" is used as in "unload the cargo for the truck". This usage is slightly less orthodox, but still makes quite some sense if you take it as a personification, treating the truck as if it has a mind, but cannot unload the cargo by itself, so you need to unload the cargo for it. This usage appears because everyone handles objects in their mind differently, and there are people who are used to this sort of personification, as demonstrated when someone consoles an object if the object falls from their hands or a table (mostly kids as adults don't express their thoughts as much. I cannot be sure, but this may be related to empathy). This kind of usage is further proliferated by human interactions in the society, where people tend to be affected and pick up others' language usage patterns.