Fermentools is the only manufacturer that produces real glass fermentation weights specifically designed to fit the wide-mouth Mason jar, measuring 2 ¾ inches in diameter and 10mm thick.
Other companies do not have glass weights, or they have them in odd sizes. If your weight is too small, you have to use more than one to get the job done. If the weight is to thick, you cannot remove them from the jar without tipping the jar over. People come up with some creative things to keep their food submerged, but nothing works as effectively as a heavy glass weight.
Not only are the glass weights needed to keep the ferment below the brine level, but they also can be used for displacement. For instance, if you make a small batch of something that does not fill up the jar, you can simply use extra fermentation weights to occupy the void left in the top of the jar. Since a small ferment will only off gas a small amount, this practice fills the extra space with glass, leaving only a small amount of oxygen remaining to be displaced by your ferment.
Fermentation Weights are the single most expensive component of the kit, but they are extremely convenient to have on hand.
The fermenting weights are made of soda lime glass, and are used to either press down on veggies (like sour krout), to press air pocket out of it over time, or to keep veggies submerged under the brine. Some veggies will float, and others will become buoyant after gas bubbles cling to them. The weight will keep them submerged under the brine, where they belong.