RainmanP

It depends on your terrain and how you ride a bike. It is quite flat here in New Orleans so for my 22 mile round trip commute I often ride a singlespeed or even my fixed gear. When I ride a multi-speed bike I seldom shift gears. Obviously if one lives and rides in hilly or mountainous terrain one needs the option of low gears. For routine riding these needs can often be met with relatively few gears.

However, when one has focused on developing a smooth, round pedal stroke and cadence in the 90-100 RPM range it becomes necessary to have numerous gears to select from to be able to maintain the same cadence and relative pedalling effort with slight changes in speed when riding in a group. Maintaining this consistency is important to riding efficiency, and becomes particularly important on long, brisk group rides or races. If you don't have close gear ratios and are forced to use a harder or easier gear than you need, your pedalling effiency will suffer. Too hard a gear, your legs will get tired from having to "push" too hard. Too easy and your legs might be fine, but you will begin to get out of breath because you are having to spin faster than you are used to. This is why you see lots of gears on bikes, for those who race or like to participate in long, brisk group rides. Most of these people not only want many gears, but want them close in gear ratio, ie, 1-tooth increments in back, like 12-13-14-15-16-17-18-19-21, or 13-23, etc.

Offroad mountain bikers have different needs. They may encounter flats or short, very steep spots. So they prefer 3 small chainrings up front and wider gear ratios in the back, like 11-12-14-16-18-21-24-28-32, or 12-34. Again, this allows them to find the right gear to maintain their preferred cadence and effort.

Noodling around town one is all you need.
FWIW,
Raymond