The purpose of a Non-Disclosure Agreement (NDA for short) is to protect your confidential information and trade secrets.
When you work with anyone - employees or independent contractors - you're likely to share private information with them. In fact, in the online world it's pretty necessary to share things like logins, passwords, client lists, business opportunities, contacts, and sales data with your staff. It's also common for you to share some of your trade secrets - maybe not everything - but in the comings and goings of running a business, your team could learn enough. And if those things got out to your competitors, it could have a direct affect on your business.
That's why you make 'em sign a non-disclosure agreement - just in case.