I think it can be helpful OR a hindrance... The hindrance is mostly if you get a bad therapist (which believe me, I've had my fair share of in dealing with anxiety).
IF you know you are transgender, exactly where you fall on the transgender spectrum, and what your goals are, then I don't think a therapist is completely necessary, HOWEVER that said, if you are planning to transition, you need to look at not only doctor requirements/government requirements for transition, but also your insurance requirements (if in the US or other places that make you have insurance/pay for healthcare). For example, I believe my insurance, which has (For now) an AMAZING trans healthcare plan, requires a therapist to diagnose you with gender dysphoria and a certain amount of therapy before especially the surgeries. I don't remember the exact requirements for the hormone therapies though.
Now, if you are uncertain about anything, I feel like finding a good therapist is in your best interest. Sure you can post in forums like this, look at videos online, talk to other people who are transgender. But your therapist will get to know you, follow your journey, and guide you through figuring things out. Like, I started out with my therapist not knowing where I am on the gender spectrum, other than not strictly female and very masculine. I still am discovering on my own a lot about myself, but as I have spent the past couple months in therapy, I've come to a deeper understanding of what transgender means for me, how to cope, deal with my anxiety that isn't even always about gender, how to accept myself and all it entails. And we're starting to pave the way into where we go from here. My journey is still far from over, but I've come a long way. And I owe a lot of it to my therapist