The benefit of using your existing nsec is the app can import your contact list and, if you run a mint, allow your followers to join it. If you'd like to test out the app, you can create another nsec first, then decide to transfer over your sats later if you want to move to your existing identity. The cashu proofs are encrypted with your nsec. The app further protects your privacy and security by deriving a dedicated wallet nsec from your personal nsec so it’s deterministically generated for ease of backup but not associated with your identity npub. Additionally, you can use a password in the derivation scheme to add another layer of security or plausible deniability.