@Kai
You can consider !:0 as a "shortcut" to the previously typed command name.

echo 1
!:0 2

is the same thing as

echo 1
echo 2

This is mostly useful for long command name. And as a generalization, you can use a similar syntax to alias any "word" of the previous command:

mkdir /tmp/x
cd !:1

Finally, it supports range:

mkdir abracadabra barbapapa chachacha dingelidon
chmod og-rx !:1-3

It can avoid lots of typing for complex and hard to spell arguments, eps. when working on the console.

@YesIKnowIT
Thanks for the clarification!
Thats really usefull.