Simple question. What exactly do Dream guides do? What is their purpose? I haven’t met mine yet, but I’m really looking forward to it!
I believe they are what you want them to be. I believe mine is a being that will help teach me dream control and help me achieve lucidness a lot more often. 
The answer to that question depends largely on what you belive a dream guide is. It also need not have just one answer especially if you believe something like I do that sometimes things can be just in your head, but others may be more.
If you believe they are all just in your head then the answer could range from: they have no purpose we just make them up, to, they are personifications of you sub concious and/or a type of self-feedback.
If you believe they are more then the answer would depend on what you see them as. It could be something like they are; angels, other souls or guiding entities unlike us.
Simple answer: dream guides guide you in the dream
They can just be dream characters that help you with something in a dream