Some communities require it. From my experience, the larger the community, the more likely they will be to require it.
There are two general concerns. The first is related to professional practice. If the planner makes an error, is there likely to be an impact that could potentially result in a claim? At first blush it may seem like the answer is no, since we do not "put a stamp" on anything. But consider what may happen if we advise the community to do something that may be inappropriate or even illegal. This does not happen often, but I have reviewed the work of other consultants who have clearly advised their clients in ways that have caused their client to act illegally. In some cases, this has had a cost of several million dollars.
The second issue is more in the line of general liability. If, while acting as a consultant for the community, you get in or cause an accident, are you covered? Do you have the capacity to defend the community against claims?
I used the firm referenced above for my insurance needs when I was an independent consultant. My cost was about $1200 per year.