Because the App Store, Google Play & Windows Phone Store require you to have it.
Especially if you collect geo-location, log server files and process the subscription payments.
SaaS Apps need to disclose what types of personal information is collected too!
How, what, when, where and why is users' personal data and financial info being collected. Why is it necessary and can they 'pick and choose'.
Is personal data only collected and to enhance user's experience and deliver more individualized content. Or is data also shared with third parties.
Security measures. How is personal data protected? Show you take data privacy seriously. Make users feel safe!
Determine how personal data may be changed, amended or deleted. Make sure every user can do that. It is their right!