It never hurts to have your own TA. Ask for recommendations from your friends - we aren’t allowed to recommend them here on CC. Once you have one, it’s true that on a river cruise there’s only limited use for OBC but if laundry isn’t included then you could use it there. Perhaps for upgraded wines and gift shop purchases or spa, if there is one onboard.
But a good TA, especially one affiliated with one of the big consortiums Signature or Virtuoso or American Express can offer hotel perks such as breakfast, upgrades, credits and my favorite - early check-in and late check-out. That comes in handy when traveling overseas. These perks do not cost you extra. A good TA has traveled extensively and can make hotel, tour, transfer recommendations and bookings along with assisting and booking air. After cultivating a longterm relationship with my TA, she has comped hotel nights, complimentary tours and lovely gifts.