When you sync Fax to Google Drive it will add a Fax folder to your Drive homepage. All copies of sent and received documents will be stored here for your convenience, so you do not have to go back to the website to access them.
You'll also now be able to send documents directly from Drive! For instructions on how to do that, see the following article:
You can also upload directly from Drive when signed into Fax! To do so choose either the 'Send a fax' or 'Sign Documents' tab, find the icon for Google drive under the 'Drag files here' box. Click to bring up a window with all of your Drive documents.