According to KYC policy, the process of a business verifying the identity of its clients and assessing their suitability, along with the potential risks of illegal intentions towards the business relationship, you may be required additional information by the Payment Gateway PayPal / Braintree for KYC verification. Additional information can be the Photo that you holding your ID card or Passport.
Verification is only required in a few cases.
Check more detail about KYC policy from this link