It is a very important piece of security because your user-name, PW and most other acct information is stored on a server outside of your PS4/PS3/PC/etc which can potentially be hacked into. As far as your acct being tied only to the one system, while farfetched it's not impossible that a hacker would also have access to methods like key generators to grant them access to play a stolen acct on a PC.

Think back to a couple of years ago when the PSN was hacked into. It was confirmed that "USERNAMES and PASSWORDS" were among the leaked info. Say that this was your XIV acct and a compromise of this nature took place on square's servers (not saying it would but theft is always possible), prior to the hack being discovered if there was no One time PW they could get right into ANY unprotected acct for which they had the info for. The One Time PW is everchanging and very difficult to verify or track making VERY VALUABLE.