Wraps?

Good afternoon, everyone. I am wondering if the wraps I see on dreads are more for looks or if they have a purpose? If there is a purpose, what is it? I'd like to know how to make long lasting ones. What is the best material? Will they cause issues, long term - from water & other elements? Thank you.

updated by @claire3: 01/13/15 10:05:25PM

If I am correct I believe I read some where on here that wraps are for decoration and aren't all that great for young dreads because it constricts and prevents proper growth. If and when you remove the wrap you'll have a very thin gap in your dread and such. Also I do believe they hold water and create mold. But I could be totally wrong as well.