How often do we go to a cafe and see the barista pressing like crazy? On that coffee that's not brewed yet, but already too black and burnt. Luckly not all the times, but way too often. Proper tamping doesn't mean pressing to death, it means ensuring that the bed of coffee lying in the filter, ready to turn into espresso, becomes smooth, uniform, and above all flat, horizontal and without holes.
Why? A slented bed of coffee, inserted in the espresso machine (no matter if electric or manual) will cause the hot water to flow more to one side (the lower one) and less to the other: the espresso will be half over-extracted (too much infusion = bitter!) and half sub-extracted (too little infusion = no aromas!). Would you like a bitter coffee without aromas? No, thanks! The same happens if there are holes on the surface: the water will likely be channeled into these (the so-called channeling), once again over-extracting the bitterness from the coffee that gets infused the most, and under-extracting aromas from coffee that is almost untouched by the water.
Well, I've spent quite a few words on this to tell you that the main virtues of a good tamper are solidity, weight balance, flatness of the tamping surface, and above all an accurate diameter that fits perfectly into the filter, leaving no room for the coffee to come out when pressed.
(a little secret: many people think that if the espresso is not concentrated and creamy enough it's because they didn't press hard enough, but that's not the case. Just grind the coffee a little finer: a slight variation makes miracles)