They can apply in any number of situations.
Whether it's to do with the union reps being required not to share sensitive information until they have to, or whether it's to do with not disclosing company secrets to the competitors that could damage the company, or whether it's to do with not disclosing share-sensitive information in publicly quoted companies, or to do with access to confidential employee information, etc etc etc.....
At the end of the day, it's an agreement. It's provided by one party as a pre-requisite to sharing certain information . It's up to the other party to decide whether they want to sign it or not. But, if they don't, then they shouldn't be surprised if they do not receive the information .
A NDA is a means to ensure confidentiality. You can ask people to respect confidential information, or put them on their honour to do so, but that's not worth anything if the verbal agreement is breached. The NDA is the legal "stick" used to enforce confidentiality if the "carrot" of appealing to better natures might not work.