Why do you need to send a transaction out with NXT to secure your address? When setting up an NXT account, it is recommended to send a transaction out in order to secure your account. I don't quite understand why it's needed. I know that if you don't send one out, your account is only 64-bit encrypted and when you send it out, it becomes 256-bit encrypted. I don't see how that makes a difference as you can still brute-force it regardless given an amount of time.

Why is it necessary to encrypt it when sending an outgoing transaction compared to just receiving one?
NXT addresses are only 64 bits long. So to find a passphrase that matches it should only take on average 2^63 times.

But once you send a transaction from the address, the address gets permanently locked to the public key you used to make the transaction. The public key is 256 bits. So now even if someone cracks the address, they can't do anything at all until they crack the public key.