TAs nowadays don’t have access to better deals than the general public for WDW resort stays.
What they can do for you completely depends on the agent. They should have a decent amount of park knowledge to help guide you in your choices. Some offer to watch for discounts, in which case it is likely they get you rebooked before you even realize the discount is live. Some will also book things like dining for you when your window opens.
The downside is an agent owns your reservation so you need to go through them for payments or changes.
If you already know what you want and you’re Disney experienced, I wouldn’t switch to using a TA now. If you want to use a TA I would try to find someone others recommend. Even within the same agency there are good TAs and lousy TAs.