I've done trips both planning on my own and using a
travel agency (Dreams to be specific) and I think for trips where you feel you are in the know and if you really like getting into the details and being on top of things, it is fun to do it on your own..... but if you are less in the know they can be super helpful
I think especially with tracking discounts using Dreams can help a lot as the one time I did use them for a WDW they helped with what the different options are and then when new discount came out they re-compared options, etc. I found I preferred to do things like booking ADRs and FPs myself, but others I am sure like having their agent do everything
When we did our
Disney cruise, I knew so much less about that compared to WDW so having an agent was super helpful for that (plus we got a nice gift basket and on board credit)