Standards are published so that testing laboratories know the proper procedures to follow in characterizing materials. For example, consider an 85-100 Pen bitumen in ASTM D946. When a testing laboratory states that a bitumen meets an 85-100 Pen grade the laboratory is confirming that it performed the tests according to the procedures in ASTM D946 and found the bitumen properties to meet the requirements for that grade.
A laboratory can provide quality test results without being a certified lab. However, a certification provides the owner/buyer of the material an expectation that equipment and procedures were used that met certain minimum quality requirements. This usually means checking equipment for proper operation and calibration, technician procedures to assess understanding and competence, and quality systems to evaluate the system used to manage equipment and technicians to produce quality results.
Ultimately the decision on whether a certified lab is needed or preferred rests with the buyer of the product (in this case, bitumen).