If you're new to planning a WDW vacation or just don't enjoy the stresses of making and managing your own bookings, FP+, ADRs etc., then using an experienced and reputpable WDW travel agent is a win-win! It will cost you nothing, will make your planning more enjoyable and efficient, and will give you a well-educated resource for any questions and concerns that you have along the way. (*Just make sure the TA you choose is well-reviewed ahead of time. We once made the mistake of using a "friend" who was a WDW TA, simply because they were a "friend," and they turned out to be so sluggish and inept that I still ended up doing everything myself, and the friendship was forever tainted...)
If you're like me, and are a somewhat-experienced WDW visitor who genuinely enjoys researching and planning (read: overplanning!) your own vacation and wants the power to modify your own booking at will, then maybe using a TA is not for you.