Slate is a form of quarried stone that acquires a slick surface over time as the surface slowly sloughs off. Because of this, it can be very slippery when wet and so is better suited for edging rather than central paths in a garden.
Certain materials such as cobblestones, gravel or soil will tend to shift as people walk on paths and the weather has its way with flower beds. Using flat pieces of slate as edging will help contain these separate areas as well as define the outer edge of your garden if you stack it several layers thick. Slate is generally 1 inch to 2 inches thick, so using at least four layers will create a nice barrier.
If your garden does not have raised beds that need containing or you have grass next to your edging and wish to mow over it, then layering your slate edging with one simple layer creates an organised look while allowing you to access the grass beside it.
Turning a piece of slate on its side so that it is vertical instead of horizontal and parallel to the ground is a way to create a low retaining border for sections of your garden. This holds soil in place sturdily. Gordon Hayward, author of "Garden Paths," recommends setting foot-wide stones on edge so they are 3 or 4 inches above finish grade.