TEL was 'invented' by Thomas Midgley Jr as an anti-knock additive to petrol (gasoline), effectively increasing its Octane Number and reducing the propensity of the fuel charge to detonate prematurely in the cylinder. This function is performed in modern unleaded fuels by the addition of methyl tert-butyl ether, which is the dominant sharp odour note in modern unleaded petrol.
In a staggering twist of fate, Midgley also pioneered the development and use of chlorofluorocarbons which, as I am sure we all know, are responsible for ozone depletion in the upper atmosphere.
Fair enough, the law of unintended consequences might be applicable for CFC's but he really should have known damn better about the tetraethyl lead!