When you first decide that you want to go on a cruise, you will consider a travel agent. We did this ourselves. I had already done all of the research online for the cruise knowing which one we wanted to take and all the information on it. We contacted a travel agent thinking that it would get us a better deal. She actually gave me a higher price than the website showed. When I told her what the site showed, she thanked me and said she was sure she could get that price. Sounds to me like sometimes you will spend even more money with a travel agent.
Basically the travel agent has information for you and sets everything up along the way. They do not get you better prices. They can get the exact same prices that you see on Carnival's website. They will go over all of the information on the site that you can see for yourself. They will tell you things such as food is included, they have formal nights, and more. This is all stuff you can read on Carnival's website.
When it comes to booking your cruise, the travel agent will book it for you. This means if you have any problems or questions Carnival cannot help you at all. You must contact your travel agent. Carnival is open 24 hours a day. Your travel agent is probably not open these hours. Carnival has amazing customer service that is there to help you out. If you call and give them your information, they will not be able to assist you at all since you booked with a travel agent.
We booked our cruise directly through Carnival. We got the same price as we would have received with a travel agent, but we can now do everything ourselves. I am able to call Carnival 24 hours a day and 7 days a week with any questions. I can change my room or cruise dates through them directly. I prefer to research everything myself before the trip.
If you have already done all the research, why pay someone else to do it for you again? The only reason I see using a travel agent as helpful for a cruise would be if you want to have them plan everything for you. You can call and say I want a cruise these dates and they will book you. You will not be able to research what is best for you. It is more time effective to do the work yourself.