Particularly for lever espresso machine, a group with higher mass allows you to run higher group temperatures without the group temperature breaking out of equilibrium and getting hotter and hotter when consecutive shots are pulled, assuming you get the integration of the group with the boiler right
However, moving to a machine with a group temperature that is in line with the top end commercial machines has revealed to us that we have been roasting too dark. We are now working hard to re-profile our roasts.