Some of the espresso machines are shipped default with a higher bar extraction pressure for extracting with a pressurized basket. However, if you are using a non-pressurized basket, the higher-pressure extraction pushes out your extraction much faster resulting user need to grind finer to slow down the extraction. As a result, you might be getting espresso which is off balanced, and the more so on the harsher side. This also leads to espresso channelling occurrences.
In many proven studies, espressos are at their most balanced stages when extracted at 9 bar pressure. In fact, 9 bar is exactly the pressure you’ll find in most commercial grade espresso machines, such as the ones commonly found in cafes.
In summary, if you are not using the pressurized basket and looking to improve your shot extraction, you should go for the 9 bar extraction mod.