Completing partially downloaded file
Suppose you are downloading a very large file, after 80% downloading your internet connection is interrupted due to some reason and file is not completely downloaded. In this case, instead of starting a whole download again, just restart downloading from where it stopped with the help of c option.
wget -c https://www.google.com/doodles/australia-day-2015
File name with append .1
When a file is already downloaded or partially downloaded, on downloading the same file again (without c option) file name will be saved with an append .1. If a file with .1 already exist, then file will be saved with append .2
Look at the above snapshot, File was already downloaded. On downloading it again, the file name is appended with .1