If you drive at 3000 rpm rather 2000 rpm at the same vehicle speed -- you're in the wrong gear.
Driving at constant speed doesn't require a lot of power, so you upshift to drop the RPM and cruise along for quiet and economy. When you want to accelerate, you downshift, and increase RPM, because higher RPM is generally more power. (The engine's peak power is usually not exactly at redline, but somewhere between a little to somewhat lower, depending on the design.) Once you've accelerated to where you want to be, you upshift again.
You don't drive around with a particular constant RPM as a target.