Just to throw my two bits in...
In general, we normally like to research and book our travel ourselves direct. We only go through a TA if it's a quirky itinerary or if it's a deal through a consolidator.
However, for our four cruises, we've always gone through a TA. Main reason being is that there's no cost advantage/disadvantage in doing so, so why not have a second set of eyes for your trip. YMMV depending on how good of a TA you find but person experience has been...
We've always had to find & notify our TA about price discounts but they've been pretty good about following up with Royal Caribbean.
They've provided us some perks in our cabin
Never really got any nuggets of info that we found really handy. [Part of this tho is that we spend countless hours researching our trips as it is part of the fun. For someone where this is a chore, I can see where TA's can fill in the gaps.]