How is a NXT account address generated? Why do we need to send out a transaction in NXT to secure the address?
It is confusing, especially since the Whitepaper:Nxt is misleading when it states:

When an account is accessed by a secret passphrase for the very first time, it is not secured by a public key. When the first outgoing transaction from an account is made, the 256-bit public key derived from the passphrase is stored on the blockchain, and this secures the account.

It is not possible to send a transaction from a brand new account since it has a zero NXT balance and the minimum transaction fee is 1 NXT. Therefore, the only way to secure a new account is to fund it from some other account using the account ID together with the public key, which is then recorded in the blockchain.

The forms used for sending NXT, even on the major exchanges, now provide a field for the public key, in case it is the first transaction to a brand new account. Otherwise, the public key field can be left empty.

The reason why this one-time extra step is necessary is because the 8-byte account ID is much shorter than the 32-byte public key it is derived from. There are many secret passphrase/public key pairs that reduce to the same account ID. But once a particular public key is associated with an account ID by storing it in the blockchain, no other secret passphrase that generates a different public key can access that account.

The developers of NXT initially chose to use an 8-byte account ID, thinking that it is more convenient than a 25-byte bitcoin address. But the trade-off is that collisions are much more likely. It was later decided that the risk of a collision was too great, but by then it was too late to increase the account ID length without a major disruption. So, the above solution was devised, which preserves the convenience factor while solving the collision problem, all at the cost of a one-time but awkward additional step. 

Unfortunately, this extra step raises the entry barrier for NXT, lowering its adoption rate.