Kinda funny thoughts on that........
On my manual shift cars, I never really paid much attention to the tach. I could tell by feel and sound when the engine / road speed dictated the next gear.
On my automatic transmission equipped cars, getting to know the usual tach reading while in top gear (ratio) at various highway road speeds was somewhat useful in determining if the transmission was malfunctioning ("slipping").
Either way, I mostly saw the tach as "eye candy", , , , since I have the "mechanic's ears" to tell when something's amiss