This is an additional security measure to protect the Access app from unauthorized access. And even if an authorized person were to access the Access app, they cannot launch it because they do not know the correct PIN.
If you deactivate your iPhone's screen lock after installing and activating the Access app, the Access app will no longer work. For security reasons, re-enabling the screen lock is not enough to be able to use the Access app again. You have to go through the activation process again (see Activation).