Finally, the tach can be used as a raw assessment of the load you're putting on an engine and you can optimize your driving habits according to that.
Perfect example of this are the people who drive with an automatic and somehow manage to stay just below a shift point much of the time, meaning that the engine is at a higher RPM than it needs to be and could be consuming more fuel and wearing faster as a result.
...this doesn't really tally, especially in modern automatics with electronic gearboxes. Most engines are optimally efficient at some key number of revs, and the gearbox strives to keep the car in that range.
Perfect example of this are the people who drive with an automatic and somehow manage to stay just below a shift point much of the time, meaning that the engine is at a higher RPM than it needs to be and could be consuming more fuel and wearing faster as a result.