What is the significance of talking to "another guy" to Crooks?
When you read the way that Crooks is described to George as he first arrives at the ranch with Lennie, as a modern reader, you may get to experience a shock at the inhumane way that Crooks is not just treated, but also valued at the ranch.
All the farmhands agree in general that Crooks is not a bad man. It is also generally agreed that he is hardworking, loyal to the ranch, and that he keeps books for a reason. However, they also agree on other things: Since he is black, he needs to stay outside of the territory of the other people. He is expected to be in isolation. He is expected not to talk or share or communicate with anyone else.
This is essentially because, since he is from a different race, he is assumed to be altogether different. Moreover, these were the years of the Great Depression, when racial relations were even more tense and divisive than they are now. Years before the riots that we know of today, of the Civil Rights Movement, and anything in between, there was a period of complete racial division and intolerance. Crooks lived in that time period.
[During Chrismas celebration] They let the n**ger come that night. Little skinner name of Smitty took after the n**ger. [...] The guys wouldn't let him use his feet, so the n**ger got him. If he coulda used his feet, Smitty says he woulda killed the n**ger.
Now, what does Crooks do? He develops a persona of defiance and arrogance to cover up the obvious pain that years of isolation must have caused him. Moreover, he attacks first by shooing people off his personal space.
All this said, the importance of talking "to another" is that such is a thing that, while is natural to us, to Crooks is a rare, yet welcome, event. It means giving the man back his humanity, his right to connect with others, share, and feel as part of a community. Crooks's isolation is just a stamp of how chaotic the times were; of how cruel and ignorant society could be. It was a sign that things needed to change right away. George and Lennie were the conduits through which Crooks actually re-established his line of communication with the rest of the men.