Strictly to raise the octane. You can then run higher compression and more advanced timing, and get more power from the same fuel, which means higher effieciency, so you can stretch your fuel supplies. Unfortunately, all the lead in the fuel ended up in the air and into out blood streams, where it's incredibly poisonous, especially to your brain.
Prior to lead, engines only ran 6 or 7:1 compression. With lots of lead, this can become 10:1 or higher. Compression ratio is related to thermal efficiency (look at diesels with 22:1), but I don't understand how or why.
Lead might be good for the valves, but it really gums up the engine otherwise, even the oil. I think this was a big factor in why we used to change engine oil every 3000 miles and now you can easily run 7500 to 10000.
- Patrick