I’m curious about this too. Heard it two different ways, one saying aesthetics only, and another saying it was intended to help guide the saw at the beginning of the cut. Personally I have yet to lay my thumb against it.
Perhaps I'm Just Over Eager, Better to Curb the Enthusiasm
Ron Herman explains that is has no function whatsoever; it was a design feature that was introduced simply to sell more saws. Others suggest that it provides a hook on which to attach a guard when transporting the saw. Personally, I use it to open bottles of beer.
North side of Chicago. -- "Such a long, long time to be gone; such a short time to be there."
I think Mr Sellers covered this briefly in one of his videos (no idea which one) – saying that some said you could use it to scribe a line by sticking a pencil in the nib and pinching the saw plate, but that he didn’t think that was very practical. At least I think it was Mr Sellers, I could have heard it somewhere else.