For most people, the tachometer is to make them feel that they have bought a
sporty car.
Certainly, you should avoid driving above the red line if you want the car
to have a
chance of lasting.  And, if you keep the engine in its most economical RPM
range (which
you will probably have to determine yourself), it can help you conserve
fuel.

Drove my father-in.law's little Sonoma pickup with the 4 cylinder engine the
other day.
Even it has a tach.  I noticed that this little gas gobbler revs up to 5000
or more going up
hills.

Deciding to have a tachometer is a bit like deciding weather or not you
would like to have oil pressure and temperature gages rather than warning
lights. The tachometer provides information that you can use if you want to,
but certainly isn't necessary to have, unless you plan on using the
information to monitor you driving habits and conditions.  Occasionally you
may find it handy to reassure yourself that the engine is indeed idling at
the proper rpm even though is sounds a little different for some reason or
another. It can come in handy as a built in diagnostic tool. For instance,
if you were checking you A/C pressures and the repair manual said to operate
the engine at 2,000 rpm while taking a reading, you would not have to hook
up a hand held tachometer because you could just use the one already built
in to the instrument panel. You could possibly improve you fuel economy by
driving in a manner that allowed your transmission to up-shift at lower
rpms. You might do that by listening to the engine to determine approximate
rpms, but having a tachometer would make it easier and more precise.  Having
a tachometer can offer a little extra piece of mind and add to the overall
enjoyment of driving.

It is visual entertainment for the otherwise bored driver.  You can
watch the needle increase to 3,000 rpm and drop back by 500 rpm as the
5 speed automatic with torque convertor lockup changes gears repeatedly
in traffic.  Kind of like watching a flickering lights on a cable
modem.  Or, as you are cruising down the interstate at 70mph you can
marvel at how the needle stays pegged at 2,700 rpm mile after mile
after mile after mile.

It is also visual proof that the 4 door sedan is somehow a sporty car
or it can provide the driver with proof he is a sporty driver.

Not really.  If you car is an automatic then all you have to do is to
concentrate on holding as steady a speed as possible, use engine
braking where possible and avoid rapid starts.  And keep the tire
pressure set properly and at the high recommendation in all 4 wheels.

If you have an increasingly rare manual transmission then the above
rules automatics apply, plus you should upshift as quickly as possible,
consistent with acceleration needs.

a tachometer is a status symbol. shows the number of complete
revolutions the engine makes each minute it is running.

some use the tach as a shifting guide. And as a gauge to determine at
what rpm / minute vs mph - for best efficiency..also indicates the idle
speed.

For the unknowing: A tach is a useful tool when you have an automatic
when towing. When I go up a hill and the automatic downshifts out of
OD, I can see my RPM and slow down to keep the RPM within a
safe/economical range.

Other uses are diagnostics, economy and shift ranges at different
speeds.

Hank