The PIN adds a layer of verification that can ensure no one can get access to your account, even if they have access to your phone.
For eg - If your phone is also used by others, and if they launch the CoinDCX app, they will be asked to enter a 6 digit PIN to get access. Since the 6 digit PIN is only known to you, they will not be able to access your CoinDCX account on the app.
