A stopcock on the bottom allows us to drain off the lower layer, separating it from the upper layer. One important use is to concentrate samples. A liter of industrial waste water might contain 2 mg of chloroform. If our instrument can detect 20 mg we'd need a 20 ml sample. If we extracted this liter of water with 1 ml of hexane we'd find all the chloroform in the hexane layer.
Some solids have a strong affinity for specific materials.
Activated charcoal is one of the most widely used materials. It has little tendency to adsorb nitrogen, oxygen or water but it will adsorb many organic compounds. A charcoal filter on a water line will remove most of the chlorine and many of the materials that produce bad taste, even at very low levels. We usually use charcoal and other adsorbents to remove unwanted materials. However, it is also possible to use charcoal to collect and isolate something we want to measure.