You do not need it. However, if you added the DOCTYPE and things started displaying incorrectly, it's very likely that your code is malformed. In other words, you coded something wrong, and with the strict standards of the DOCTYPE, it stands out.
You should validate your webpage here to see what is wrong:
http://validator.w3.org/
More info about DOCTYPE:
http://www.w3schools.com/tags/tag_DOCTYPE.asp