Imagine the following situation: you would like to clone textmate repo and make a typo in a word git ('_' is your cursor position):
$ gir clone https://github.com/textmate/textmate _
So you'd like to correct it and... Well, the vast majority of programmers that I know will hit left arrow key over 30 times to get there because they don't know that:
Ctrl-A returns cursor to the beginning of current line,
Ctrl-E moves cursor to the end of the line,
Ctrl-K deletes the rest of the line, beginning from the cursor.
Those shortcuts work on OSX almost everywhere, for example in Safari address bar.