I tend to agree that the benefit of stuttering is not clear from an evolutionary perspective but some other psychiatric “disorders” such as ADHD do have a clear evolutionary benefit. People with ADHD are typically good at seeing the big picture and can have hyper focus on things that really interest them. Focusing on things that were really important could clearly be helpful in primitive societies. Anxiety is the most obvious one though. While it may not be fun for the anxious person, the person who stayed up at night worrying if a tiger was going to come kill their kids could definitely increase the probability their kids survived.