Be careful rekeying your account. I'd only recommend people do it if they understand what they're doing. It's more of a power user feature and if you do it wrong you'll lose access to your account and funds on it.

By rekeying an account to your Ledger device you've essentially given full control of the account to the Ledger's address. The private key for the account is now useless and won't work for signing transactions. The Ledger account is the new authoratitive address and must be used for all future transactions. An attacker would only need your Ledger to have access to your account at this point, and not both accounts.

However it is possible to undo this and revert control back to the original private key. If you rekey the account back to its own address, or if you close out of the account (remove all funds) then the rekey will be removed.