Adding an extraneous weight to your workout (a sandbag in this case) increases the load on your muscles which is a little like working out in Goku's Hyperbolic Time Chamber. You get stronger, faster. This is exactly what Bag & Tag is designed to do. Each exercise is designed to target specific muscle groups and some recruit a number or related muscle groups. You can use a sandbag of any weight but an EC grade one gets you ...well, EC!
Extra Credit: Use 16kg+ (32+ Lb) sandbag.