Roman valued verism in their art for several reasons. One being that the Romans held their lineage in high esteem. If you came from a good, rich (not just wealthy) line of Roman citizens, then wouldn't you want to show off every mole, wart, and wrinkle that makes you part of that lineage?? Well, the Romans certainly thought so!
More importantly though, were the Romans' views on individuality. The Romans believed that a person's individuality (strenghs, weaknesses, and personality included) were wholly connected to a person's facial features. This is why so many Roman portraits are only from the neck up!
The Greeks' sculptures focused mainly on idealistic religious figures: Venus, Apollo, Zeus, and Herkales. Romans however focused more on secular art--commissioned art. All the Greek idols look the same: a canon of immortal beauty! Roman emperors, senators, and generals didn't want their portraits to all blend together. Looking at a Roman portrait bust is like looking into the life of that particular person. What trials and tribulations made all those wrinkles? From where did he receive that scars? It's hard to read a blank face, which is why Romans put so much "true to life" into their art.