I am worthless, yet when added to others I add worth. I confounded the ancient Greeks, yet I changed men’s understanding. What am I?

Thanks for clarifying. I was about to say that we got it from the Arabs.

However, zero isn't the only thing that has the ability to change things. Tom Lehrer said the same thing about silent 'e'.

Oh HSAA, hello!

Yes we have just been discussing this long-time quandary of zero...         Also those pesky negative numbers, so impossible of concept.

And now, we are asked to accept the square root of (-1) !!!                           This strange little i, so impossible...yet it's needed to work with electricity and things like that...so HSAA it's already a puzzle!