Glass Daleks were a form of Dalek variant. While the first known Glass Dalek simply owed its unique casing to its elevated position in the Dalek hierarchy on Skaro, other, baser Glass Dalek casings were later created by Davros for the reproduction of his Imperial Daleks. These were designed to turn humans into Daleks.
In a retelling of his first encounter with the Daleks and the First Doctor, Ian Chesterton claimed a Dalek with a glass casing acted as the leader of the Dalek City on Skaro. The Thals hacked it to pieces during an assault on the City. (PROSE: Doctor Who in an Exciting Adventure with the Daleks) By another account, the leader of the Dalek City during the Thal-Dalek battle was the Black Dalek Supreme. (AUDIO: Return to Skaro)
In Tranquil Repose on Necros, Davros used the Glass Daleks as part of his experimentation. By placing a cryogenically-frozen head into the casing, the head would mutate into the deadly creatures that made up the Imperial Daleks. When the head had become another Dalek, the Glass Dalek casing changed into that of an Imperial Dalek and another glass casing took a new head. Arthur Stengos almost became a Dalek before his daughter, Natasha, on his instructions, killed him. (TV: Revelation of the Daleks)
During the Last Great Time War, the Daleks gathered up humans and placed them in Glass Dalek casings which acted as incubators while the human inside was mutated into a Dalek, after which they would be placed into a standard Dalek casing. These Daleks were used as foot soldiers and cannon fodder as they were viewed as inferior and more expendable than pure-bred Daleks. The War Doctor and Cinder found five of these casings in a Dalek laboratory in one of their saucers at a temporary Dalek base in the city of Andor on Moldox. Cinder tore up the cables, allowing the people inside the casings to die in peace. (PROSE: Engines of War)