First I will list the 11 curative factors in group counselling and I will use the last 2 to answer the question:
The assumption that we are alone in our sadness or problems could be really overwhelming, in group therapy the more vulnerable you are, the more likely others empathize with you, many people on therapy think that no one gets them. Sharing their experiences and problems is helpful because they realize that other people can understand their feelings and could maybe be going through the same, and this can be soothing.
By sharing and being part of a group therapy, the group members will begin to be themselves, they will interact with each other like they interact in the outside world, so over time the members of the group will start to show their bad habits in terms of adaptive behaviors, and the group can help to be aware of these behaviors, which is the first step to change them.