If a brick patio were built directly on top the soil, it would settle unevenly over time and lead to a bumpy surface. Topsoil is a variable and compressible material and is subject to shifting and moving under the weight of the bricks and people walking over the surface. In contrast, gravel is a uniform material that does not compress or shift, making it the ideal material to use for the foundation of a brick patio. Topsoil must be removed from the patio area so it can be built over firm subsoil, which is then pre-compacted before laying a gravel foundation for the bricks.
Since a brick patio is typically built as a permeable surface without the use of mortar between the bricks, gravel is needed underneath to allow water to drain through when it rains. The gravel serves as a porous storage space to quickly drain water from the surface in a heavy rain, after which the water slowly seeps into the soil beneath the gravel. Otherwise, there would be a risk of puddling on the surface that could stain the bricks and create a dangerously slick surface.
Other Reasons to Use Gravel
Bricks can easily be dislodged by the roots of trees growing near the patio. The use of gravel discourages roots from invading the area because there is little in the way of moisture or nutrients, and the material is physically difficult for roots to penetrate. In cold climates where the ground freezes regularly in the winter, a deep base of gravel under the bricks prevents them from being dislodged by "frost heaves" in the annual cycle of freezing and thawing. For this to be effective, the gravel must extend below the average depth of frozen soil in the area.
How Much Gravel to Use
A minimum of 4 inches of gravel is necessary to form a stable base under a brick patio, although circumstances may require more. If the patio is going to be used to support more than the weight of normal foot traffic, 6 inches or more should be used. 12 inches of gravel creates a foundation firm enough to drive over, although it must be spread and compacted in layers. More or less gravel can be used to make the final patio surface at the level that is desired. For example, it is often necessary to remove up to 8 to 12 inches of topsoil in order to build on stable subsoil -- this can be replaced with an equivalent amount gravel so the patio surface is even with the original grade.