www.costcotravel.com IS a TA. I like them b/c of their cash card at the end of a
DCL cruise (and their OBC for a
Royal cruise)
If you are a newbie at cruising, or cruising DCL...if you (and any other adult in your household) are busy all the time and have no time to research...and many other reasons you might need help...then find a good, personal TA who is happy to take your calls and emails and answer you and help you.
If you are a person who might want to change things around a lot, make sure you get a TA who is very reachable. Or just train yourself to not think about changing things when the TA is closed.
People have different definitions of having "control" of a reservation.
With a TA you cannot call the cruiseline directly for things involving money. If you want to change your stateroom or add transportation, you have to call your TA who then calls the cruiseline *for you*. For ME that = less direct control.

For others, a phonecall is a phone call. Since I use Costco, and so far when I've needed them to make a call for me, they *keep me on the phone* while they call DCL or Royal. That means I'm on the phone with the TA *and* there on the line with the cruiseline, and that means double the time on the phone.
With a TA you *can* call for things like requesting distilled water for a CPAP machine. (I tend to not enter my cruise info on the phone, and just start off with "this is what I need to do" and THEN tell them my info. If I start with my cruise info, they don't want to think about IF they can help and they just say they can't) You do online checkin, you can make dining arrangements, you book Port Adventures, etc. Those can all be done by you.
There's really no way to prove if a TA can help in a way that one can't help one's self, if a problem arises. I mean, there's no way to try it both ways, so you can't prove that a TA did some amazing thing and that the cruiser couldn't have gotten it done.
the TA will give you perks back.
Not all TAs. Not by a long shot. If you go with some small at-home TA who doesn't want to give up any of his/her commission, you're not getting perks. That's one of the things that needs to be determined BEFORE using a TA.