The biggest mistake new community builders make is not having a clear purpose.

I second this. Most of the failed communities are due to the lack of clarity. If the builder lacks clarity, new members will also be confused about what value they're getting from this community and they will leave the community

True but still the community manager can help get a clear purpose for the community

Sometimes it happens that there is a clear goal, but the motivation disappears at the initial stages. One must always move forward despite the difficulties in business. Only then will you be successful.