In Ayahuasca ceremonies I attended, the sitters take a much smaller dosage to connect them into the ritual space. Their job is not to judge your sanity, but to keep you from harming yourself, or to attend to some needs (such as, getting to the bathroom). They are usually experienced with Ayahuasca, and know how to be guided by the plant spirit of Ayahuasca while bringing in common sense. Being experienced, they know what it is like to explore this, to give soqce when needed and when and how to help. In the group I was with, they often recieve training.

Like the one time I started talking with the consciousness of fire via the (convenient) camp fire started outside. I had learned how to progressively acclimate to greater degrees of heat (something useful if you have ever experienced kundalini arousal). I stuck my foot on the top of the fire to try to wake up the energy channels there (something I had only managed to pull off three years after this experience outside of ceremony space using Taoist neigong training).

The sitter with me was concerned but let me continue.

One of the things with working with this consciousness is that inhibations burn away. Things that seem risky now seem like fun.

The Fire invited me to hug the Fire and get closer. A different guide went "no, don't do that".

Then the sitter said, "Ok, I smell bacon" (meaning, my flesh is cooking) "get your feet off the fire"

I went "awww" and complied.

I'm not saying they're 100% required but I've always been pro-trip sitters; preferably someone who's been there before that you're comfortable with. It can also depend on where you are / what you're doing. If you're at home vs. at a festival or out camping.

It's nice to be able to turn to someone and say, "Hey, let's go get a Powerade." and not have to worry about how to make that happen.