Insulating the suction line is done to prevent condensate from developing and dripping off. This condensate drip can cause water damage to the material under it or cause a slip hazard for people walking below it.
Another very important reason to insulate the suction line, which is sometimes overlooked, is to limit the amount of heat added to the refrigerant as it travels back to the compressor. This additional heat will cause the system to reject unnecessary heat without any benefit. In addition, if too much heat is added it may cause the compressor to run too hot. A well-insulated suction line can save money by reducing both energy consumption and equipment breakdowns.