Why am I being asked to sign an NDA?

To protect trade secrets and confidential information, one essential element is that the material is actually kept a secret, at least from the general public. An employer (or anyone else) who wants to maintain their trade secret will want you to promise to maintain the confidentiality of their trade secrets. You will see this as part of a nondisclosure agreement or a nondisclosure provision in another agreement, often called an NDA or a confidentiality provision.