The Federal Clean Water Act has (or had the last I knew..) a Citizen's Suit provision where individuals can assume legal standing and sue polluters for loss of recreational opportunities or use of a waterway due to non-compliance and exceeding discharge limts, etc. Usually, these actions are undertaken by a group who provides an attorney(s) to argue the individual citizens case if it ever gets to court, which it most often does not.
The main benefit of this approach when dealing with municipalities that are out of compliance is to serve to break up funding bottlenecks and issues which are the most common reason discharges are not meeting standards. In pollution cases, it has been very common for the courts to issue consent decrees which recognize the current inability of the municipality to afford the upgrades and gives them x amount of time to get it fixed. In the meantime, the discharge can continue. The problem with consent decrees is that that often get extended again and again and the problem never gets fixed. Things just drag on and on.
Filing 60 notice of intent to bring a citizen's suit under CWA can serve to breakup the inertia and get things moving again. The allowable fines under CWA are (or were..) up to $25K per day per discharge parameter out of compliance. This can add up to a pile of $ real quick and tends to get people's attention. But generally, these cases actually go to court pretty rarely. A settlement is reached, the money is found somewhere and more often than not, things get fixed sooner rather than later.
So, that's the value of these sorts of actions...