Tough question 
In general, those who believe spirit guides are genuine, reason they are some sort of higher beings who help you on various levels, through the use of dreams. They help you when you’re in danger, they show you how to make difficult decisions irl so you follow the right path, etc… but all this happens mostly in the symbolical language of dreams. Some spirit guides are animals, ancient shamans or fantasy creatures. But they can also be just ordinary DCs. Their shape often correlates with your being and/or the nature of their message to you.
Ofcourse, other people just think they’re just DCs, products of your own imagination. Hence the big discussion
As Basilus West said, you can find more information about it (and discuss it) in the BIG Spirit Guide Topic - part 2. It can be found here: https://community.ld4all.com/t/the-big-spirit-guide-topic-part-ii/8327