This letter is for those situations that involved a complicated set of events and relationships, voluminous paperwork, gray areas of law, and other circumstances that require your lawyer's extra attention.
In some cases, a stern letter from a lawyer works cheaper and faster than filing a lawsuit. It shows that you are serious enough to have gone that far, and if they run it by their own lawyer, they may be advised to just pay you. It is not advisable in an eviction situation, because you tip them off that you have us to help you, and we lose the element of surprise as your secret weapon.
After some consultation with you, review of the documents, some research, and careful drafting, you will have a letter that clearly sets out your position and demands, from your assigned lawyer. If they ignore it, at least you tried, and the lawsuit comes next. If they accept it, mission accomplished. It at least opens the door to negotiation, and you could reach an acceptable middle ground.