Glass Screens for a Vaporizer
Glass screens create a layer or barrier between the red hot coil and the herbs. Without the direct contact of the coil, the herbs are only heated by the hot air and the hot glass but there is no combustion so they herb will only brown.
Not all vapes are the same. True vaporization is when herbs are heated by the hot air and brown as vapor is created but the herbs never turn to ash. That is the healthy vaping that replaces the act of smoking. But some vapes are created to make smoking more convenient. The AGO vape pen
combusts the herbs and the experience is like smoking a bowl but you can do this with the press of a button without a lighter, while snowboarding down a mountain if you want. But, you can also also buy glass screens with the vape to achieve True Vaporization and many users do this.
Glass Screens for Pipes
Sometimes a glass bowl or pipe has a hole on the bottom that is a little too large for your liking. If you break buds up by hand this may not be an issue, but if you like to grind your herbs than you will likely have a bit of loss down that hole on the initial packing and as you light and take a hit. The glass screen serves as a barrier that prevents the ground herbs from going down into the bowl head, but also does not limit the airflow when taking a puff.
Many users prefer a glass screen just because it slows the build up of residue in a bowls head. The build up of residue will restrict airflow and eventually make every smoke session and pain to deal with. The screen can also be picked out after some use and replaced or cleaned and placed back into the bowl. That is easier than the normal thorough glass pipe cleaning.