Etags vs. Last Modified Header

[trx]

Hello ,
After reading online sources (Yahoo, Google etc.) about Etags, I'm still NOT sure I really understand about why Etags were created and what websites can gain from it. It would seem that the "Last-Modified" header should do the trick to determine when a static resource needs to be downloaded back to the client machine and cached again or NOT; so in what case does that method fails and Etags need/can to be used? Maybe there's a scenario that I am missing that would be good to use Etags!???!

Thanks advance.

Regards,
TRX