Bentonite is a clay composed of very small particles, which makes this material extremely plastic. These fine bentonite particles stay in suspension in water, lending it to function as a suspending agent in glazes. Bentonite can also be used as a glaze thickener in small amounts and can also be used to thicken glazes if you do not want to use an organic compound (such as CMC or Gum Arabic).
Adding bentonite to a clay body in small amounts of about 2% will also add strength to green and dry clay as it binds clay together strongly. Bentonite can also increase the plasticity of a clay body, but this addition can also increase the shrinkage of the body. Bentonite can also decrease the drying rate. High quantities of bentonite however can result in an increase in cracking while drying due to the extremely small size of the particles, so make sure to test the percentage that you add to a custom clay body.