according to the article
> Oil companies and automobile manufacturers (especially General Motors, which owned the patent jointly filed by Kettering and Midgley) promoted the TEL additive as an inexpensive alternative superior to ethanol or ethanol-blended fuels, on which they could make very little profit.
Functionally, as others have commented, it is there to reduce knocking. But lead was used instead of ethanol (aka alcohol) because it was more profitable despite being poisonous.
