Because, apparently, without them, sentences would just end in mid-air.
This is a thing of great importance, don't you agree?
It's a matter of punctuation, really. Without periods, all our writing would be one long, confusing mess.
Imagine the literary world without periods. It's a scary thought.
Go Why Do We Need Semi-Colons Then? to explore more grammatical conundrums.
Or, if you're feeling adventurous, Why Don't We Need Colons Instead Of Periods? to see how the other side of the punctuation fence lives.