I don't get it, myself.

To me a co-dependent is an enabler to the person(s) who have issues/addictions.

I don't think group therapy is necessary for everyone.
People who are socially awkward, phobic or have addictions,
often enjoy them. But if your problems are more central to YOU then one on one can be more productive (but also more expensive).

Some therapists use transference heavily in one on one.
If yours does this, then the experience would be far different than your group style.

I was in therapy in the past for childhood abuse issues. One on one was useful for me to learn how to reframe and reduce PTSD. Listening to others' long stories and struggles, I found depressing, and useless. Remember the therapists tend to structure the group so there is at least one strong person in it.
If that is YOU then you don't get much benefit from it at all...you just enable the leader better. And you pay for that priviledge.