SLA stands for Service Level Agreement and it can play a significant role in service-oriented business. It is not mandatory or required for a business to function, but using this can bring intangible value to the organization, help establish good work routines, and increase productivity and company's results as time goes by, projecting also positively on the overall company's wellbeing.
SLAs are agreements, sometimes they can become a part of the contract with your clients, but they should not be treated as a tool of enforcement, but rather as a good will and practice from your side to meet client's requests on an agreed level. So in this sense, prepare SLAs that are going to be used with Redmine.