First I'll mention that there are other icons that are used to save, but as you've noticed the most prevalent one is the floppy disk. I've found this discussion: > The floppy disk icon has become the standard for saving files. read more
The icon nowadays represents the concept of saving more than it represents floppy disks anyway. Originally the floppy disk was probably the first widespread method of saving work and transporting it elsewhere. For awhile as tech advanced there was a quite a few different competitors and even revised versions of floppies. read more
But while that physical relationship of saving files to floppy disk may have stopped, the mental and culturally engrained act of saving files using that floppy disk _icon_ has remained (hence it remains intuitive on its own). Aside from that, I see most people use File | Save (As) now anyway (given the uncertainties of the save/disk icon). read more