When the Higher Self decides to send its fractal copy into an incarnation, it invites discarnate spirits to serve as spirit guides. The work of spirit guides is tough - sometimes they have to give you lessons, that is- bring you troubles. They do that because they understand the higher purpose of the lesson and because you asked them to do so before you dived in.
Usually people have 2-4 spirit guides. These are often spirits from your own kindred, friends who incarnated with you many times. These are often your deceased relatives, some are the ones which died before you were born. Some spirit guides stay with you for your whole life, others are changing as your lessons change. The guides are recruited by your Higher Self to help you with certain themes and life lessons. Some of them might have expertise in the major area of your work. Keep in mind that all of our spirit guides died in the past, so most of them don't have experience with modern life. You are the first one with this experience. Often there is a competition for the right to incarnate, so do cherish your opportunity to be here and thank God and any helpers for the experience.
The spirit guides are with you most of the time but not all the time. Sometimes you are on your own. It is their work to keep you alive, keep you on the path conducive for the chosen lessons, and send you the opportunities for growth. If you learn well, the troubles on your way might be minimal. But if you are stubborn and refuse to grow spiritually, the troubles might become stronger. You play a big role in creating your own reality via the Law of Attraction.
Spirit guides do much bargaining, logistics and favor exchange with other spirit guides, spirits, angels and other spirit workers involved in our world. There are many ethical and housekeeping rules and restrictions which limit their abilities. Their main power is in controlling seemingly random events. When things seem to be random, they are the ones who bring you a help, a trial, a present or an opportunity to nudge you in the direction which is chosen by your Higher Self and recorded in your life contract.
Not every random event is due to spirit guides. Some random events just happen randomly without a reason. As Freud said "sometimes a cigar is just a cigar". In many cases you create your reality and the spirit guides let it happen if it doesn't contradict the learning path.