For the user to check there is a path from your IP to the target
IP, and how 'laggy' is the data transfer.
It is very useful when you're setting up internal network. you'd
want to make sure the two computers (or your computer to a router)
are connected before you try things like file and printer
sharing.
I always use it when my browser suddenly takes very long to load
something, like google. I ping google, if no response means no
connectivity. If this persists, need to go restart the modem.
It can be used as an objective measure. For example, you want to
boast to your friend your connection has less lag. So both of you
do a ping. The one who has lesser time (in ms) wins. 
