To answer your original question as to the advantages of booking early, there are two.
>> 1. You "lock in" your choice of itinerary and sailing date. The most popular cruises do "sell out" relatively early in the booking process.
>> 2. You have a better chance of an upgrade, especially if you book a guaranteed category rather than a specific cabin.
The disadvantage is that you might miss out on promotional offers within a few months of sailing -- which usually are very generous if the bookings are light. By way of example, the "Love Boat Savers" (early booking) fare on my last cruise ("Scandanavia and Western Europe" for twelve nights aboard MV Royal Princess) was $2990 but the promotional fare was $1445 -- a 50% savings!
Bear in mind, though, that popular itineraries might not offer any promotional discounts and, further, that the lowest available fare might actually go up as the less expensive categories of cabins fill up.
If your vacation plans are fairly flexible, it's best to wait and take advantage of the discounts. If your schedule is constrained (for example, if you have to cruise during school vacation because you have school-aged children), it's much safer to book early.