When questions tell you to use this equation, it is to calculate how much heat is evolved. When heat is evolved, the reaction concerned is exothermic. When we use the equation Q = mcΔT, we would normally get a positive value. Therefore, the negative sign in the equation ΔH = -Q/n makes the calculated enthalpy change negative, showing that the reaction is exothermic, and is essential in all the questions I have seen before. I’ve never seen a question involving endothermic reactions though, but in that case the temperature would drop and the negative sign would be omitted to give you a positive enthalpy change.
Does this help?