If we can all talk to share ideas, why do we need to write them too? Writing lasts longer! But more than than, writing is a means of communicating knowledge to others rather than talking to everybody. If you need to explain what you did last summer to a friend, you can simply tell him. But what if you want to tell your entire school - or even more? Write a note once, and everyone can read it.
On top of that, writing provides proof of literacy - not only in modern times, but in ancient times as well. We are able to pass down our knowledge gained from generation to generation because of writing.
In the business world, writing is an important mode of communication because it leaves a "trail" of sorts. That is, if you told your boss that you weren't going to work on Fridays because you enjoy three-day weekends, that would be simple. However, if on Monday your boss asked where you were and he said he doesn't remember you telling him anything, if you had emailed or left a note telling him that you are not working Fridays, it would be difficult to dispute that.
And finally, writing is important - because reading is important. If nobody writes, nobody can read! By reading, you can enter entirely new worlds of make belief - or worlds that existed before you were born. By writing, you can create those new worlds and describe how the world is to others.
Now that we all see why writing is such a powerful tool, let's go pick another Writing Topic!