Why did steinbeck include the scene about killing candy's dog? this is the book mice and men

To emphasise the ongoing theme that if something/one is no longer useful they die or have no value. This is to show the harsh conditions that they live in. It also show the hierarchy of people on the ranch e.g. Candy and Curleys wife are not respected because they can't/aren't allowed to work.

Ithink that story needs some revising. hmm some academic language perhaps? and sylvia is but i do think that rainbow is spectacular