When using a traditional, commercial style portafilter, tamping your coffee becomes an important variable in the preparation of your espresso. As recommended by the "Golden Rules," a tamp pressure of 30 lbs is desirable when trying to achieve consistent, quality shots. Maintaining a consistent and specific tamping pressure of 30 lbs eliminates an entire variable from the brewing process, allowing your to modify other factors such as grind fineness and observe the differences in the shots you pull.
While it is true that the extraction time of your shots is largely determined by your tamping pressure AND the fineness of your grind, grind fineness is more difficult to measure, and as a result, more difficult to duplicate. Additionally, regardless of the fineness of your grind, tamping too lightly or too strongly can still result in shots that are too fast or too slow respectively.
So, how do you achieve consistent tamping pressure? Well, the real answer is practice. Like all elements of brewing espresso, tamping is a process that requires a bit of time to master. An easy way to get an idea of what 30 lbs of pressure feels like is to place a towel on a bathroom scale and press down, you'll be able to see just how hard you've been tamping. Alternatively, specially calibrated tampers and tamping mats exist that alert the user when 30 lbs of pressure has been achieved.