The fabric you are wondering about is actually known as a geotextile. It is a heavily woven plastic fabric that allows water to pass through and keeps your base material from migrating into your existing soils. This material is not installed to prevent weeds from squeezing up through the paver joints, however, it has been known to slow the process or in some rare cases prevent it. It is recommended to use the geotextile if the nature of your native soil is unstable, such as heavy sand, heavy clay or excessive water drainage. It stabilizes your project to prevent your base material from mixing with the native soil and compromising the solidity of the installation. Please note that products used to prevent weeds are not as strong as the geotextile