I often use guides when I have some places in my PCB where I have to align components. For example when I need to have number of components placed on one line, then I use horizontal/vertical guides. Also I use them sometimes to split my board into areas or create a simple special grid which I use during placement.
For example I used guides when I was creating boards for my video with simulations of capacitors (the guides are not visible in the video, but they helped me to create the smaller boards):