The ECL80 valve was designed and introduced by Mullard during the early 1950s to facilitate the reduction in valve count in televisions by placing two valves in one envelope that could be closely associated in such a set.  

For example, wouldn't it be nice to have a frame blocking oscillator and frame timebase in one envelope - voila, the ECL80 comprises such a system with the pentode cage designed as a frame timebase and the triode as a frame or line blocking oscillator .. or.. as an AF amplifier....or.... as a grid detector. 

Another great application is for the ECL80 to act as a high sensitivity audio amplifier with the triode cage acting as voltage amplifier with the pentode cage acting as output valve and indeed this valve found such use in contemporary single ended record player applications as well as TV usage.    Lateral progression presents other convenient uses, how about the triode cage as a grid detector and pentode cage as an audio output valve.

Although a great valve, it wasn't without it's faults, differential heater rates could cause conflicting problems in earlier televisions and the electrode system was prone to microphonics which could lead to vertical hold judders.