What is the useful product of glycolysis for the cell?

The useful products is Pyruvate

Glycolysis is a series of reaction that involves the breakdown of glucose to extract energy for cellular respiration. Furthermore, the metabolism of glucose during the process of glycolysis forms a pyruvate with the production of two molecules of ATP. Thus, pyruvate moved into mitochondria and it is used to start citric acid cycle which will yield more energy for the cell.

pyruvic acid a water-soluble liquid, C 3 H 4 O 3, important in many metabolic and fermentative processes, having an odor resembling that of acetic acid, prepared by the dehydration of tartaric acid: used chiefly in biochemical research.