Why do I look at the tachometer?

I check the rpm at run-up to see if the mag causes excessive rpm drop;
to see if carb heat is working; during take-off run to make sure the
engine is producing full power. I have a set rpm that I use for cruise
climb and another rpm setting when going in a long descent; once in the
pattern I set the rpm so that I'll bleed off the altitude as I go
around the pattern. I do look at the tach more often than the vertical
speed indicator.

Tach time would also be useful for computing endurance, (much more
reliable than the gas gauges), but where it is placed makes the numbers
difficult to read.

For impending problems, the tachometer is rather worthless, so I do
scan oil temperature, oil pressure, CHT.