A slab basement is typically much cheaper than a full basement. There is much less excavation work, less materials, and less labor needed to pour a slab than there is to dig and build a concrete basement. It can be much faster and easier to build. In areas where there is a large amount of rock or stone to remove the cost for a traditional basement can be considerably more expensive than a slab. Prices vary dramatically due to the size of the basement and amount of work necessary.
When you build a traditional basement you dig into the ground to create the space for the basement. When you do this you get closer to the groundwater level and provide a spot for water to pool. If this water-table rises for any reason there is a chance the basement can flood. If you have the basement finished for use as a room, or have anything stored inside, water can seep in and cause damage. With a slab basement you eliminate this possibility because the slab sits on the top of the ground, if there is no hole in the ground for a basement there is no chance for it to flood.
Radon is a naturally occurring gas that has been known to cause cancer. It is found in the earth and when you dig a traditional basement you increase the chance that it, and other gases, are released and can enter your home. With a slab basement you eliminate this possibility and create a safer home.
Homes that are built on a slab typically are much lower to the ground. This can be more desirable for older people who do not want to climb many stairs to get into the home.
If a home has cracks in the walls or sloping floors you can almost guarantee that the problem stems from a weak basement. A slab foundation is the most structurally stable foundation available and virtually eliminates any potential problems of shifting or settling if installed correctly.