As I understand it, the main benefit of using a clicker before giving a treat is that it makes it easier to train behaviors especially when you can't present a treat at the exact moment the desired behavior is performed. With a clicker, you can click the instant they do the right thing and it will give them a dopamine hit as if they received a treat which you then give. That makes it much easier to associate the reward with the behavior because the click anchors the reward with the precise moment/action performed, whereas giving a treat afterwards can take some time for them to figure out which action performed led to the treat.

Once they learn the behavior though the clicker isn't really needed. For example I would prompt my cat to scratch her post, then click the instant she put her paws on the post and then give a treat. That helped to speed up the realization that it was putting her paws on the post that led to the treat, whereas if you just give a treat afterwards, the dopamine hit doesn't happen until they get to the treat. Basically just time shifts the dopamine hit back in time to build the association faster. Nowadays she knows the phrase scratch your post and what to do, so I don't use the clicker anymore - it's more for training new tricks or getting training more established.