My normal answer is "Me!" But, in reality there is a big difference and it is really in the customers best interest to use a Cruise Specialist against even a "full service"agency much less the cruise line directly.
To begin with, when you call a cruise line (customer or agent), they ask for a phone number. When this is entered into their booking engine, the computer system tells the reservation person whether the caller is an agent or not and the system generates rate codes that are applicable to the caller.
Retail customers do not always get access to all the rate codes or the lowest prices although they will get access to regional, senior, past passenger promotions etc.
There are two "environments" in the booking process - individual (known as FIT) and group blocks. When an agency contracts group space for a sailing, the number of cabins in selected categories can only be sold by that agency. When the agency contracts for the space, they also "lock in" the rate. Depending on the sailing and availability of cabins, the rates can go up or down in the FIT environment but the group rate was locked in. The earlier the space has been blocked, the lower the rate the agency has for their cabin inventory.
If the rates do go down, the agency has the option to lower the pricing of anyone already booked and to book new clients at the lower rate and if the FIT rates have gone up, that agency then has a price advantage or lower price for their clients then the cruise line or other agencies. The cruise line does not hold it's own space except in the FIT environment. The pricing all agencies get when making a new booking is the current pricing in the FIT environment if they are not holding any group space. It's the same as the cruise line pricing if you book direct.
However, the agent/agency is also your advocate, not the cruise lines, so that is an advantage right there over booking direct.
Another advantage of using a cruise specialist over the cruise line directly is this. Let's suppose you call cruise line A and you actually got a reservation agent who will take time to help you select a cruise. Normally, they only want to know "how many people, what ship, what date are you looking for, what type cabin do you want (and would you like fries with that? - OK, I added that
A Cruise Specialist on the other hand, helps you determine the best cruise line and the best value for your individual situation each and every time and has access to all of them. Guess what! You are not paying any extra for this service since it is the same FIT pricing and therefore you are getting more value for your money because the agent is your advocate, not the cruise lines.
These are just a couple of reasons why booking with an agent is an advantage for the customer. And I use the term Cruise Specialist rather than travel agent because Cruise Specialists really do know more than most travel agents when it comes to the differences between cruise lines and how to find the best deals. And because the cruise specialist agency probably does more volume of business with the cruise ines and has access to more promotions (some exclusive) and all the best pricing the cruise lines have, they probably are holding more group space and can better assist the customer to make the best decision for the cruise. Full service agents who book everything all the time - well, you just can't be all things to all people all the time and be an expert in everything in my opinion.
So if you are looking to book a cruise, work with a cruise specialist. You'll be better off.