The cover letter originated from recruitment agencies who wanted to set their candidates apart from their competitors. It helps to highlight specific points that you want the interviewer to take note of in order to stand apart from your competition. Similarly in the immigration landscape the cover letter sets you apart from your competition so that you stand apart from your competition. It highlights key points that you want the immigration officer to take note of so that your overall application is taken in with high regard. 

Volunteering for community organisations is helpful to show desirable traits that the Singapore government is looking for in applicants. However it has a limited value. volunteering services alone does not guarantee success in your application. If your application is otherwise weak then do not expect that volunteering services alone with get you your PR.