You probably know, espresso pumps are generally booster pumps, not sump pumps, so they're gonna need a decent amount of inlet pressure. With the Marzocco paddle groups, the mechanical valve will open the group head to the line pressure, progressively increasing in aperture until it's all the way left, which activates the pump. This control allows for some progressive control over the pre-infusion, or the initial pressure ramp-up.
Too often, the incoming water has some pressure fluctuations, in which case a pressure regulator could help (an expansion tank would be even better).
Aside from that, I'm personally not a big fan of that type of pre-infusion. Most of the popular espresso machines on the market, including Marzoccos, feature a flow-restrictor somewhere in the group plumbing, which if placed in a useful spot, results in a progressive increase in the pressure within the extraction space. That pretty much accomplishes what pre-infusion is trying to do, and in a more consistent and reliable way. I tried for years to achieve better tasting espresso by using the line-level preinfusion with our Synesso, but that machine has heavy flow restriction too.
That said, I know that this is a debatable point. I just haven't personally experienced the benefits, despite trying.
Don't get me wrong: I LOVE paddle-actuated groups, and I'll totally buy machines with paddle groups in the future. They're much better (read: more fun) than rocker switches or buttons… but it's still about "on-off" for me.