that's my mentality! how did you guess? back to the point, vot is the benefit ov more gears?
If a car gives maximum Fuel Economy at 2000RPM
couple it with a Transmission with Unlimited Gears and u can run it from 10km/h to 200km/h while keeping the Engine at 2000RPM
If u check ur own car which has 5 Speed Gear u cannot give 2000RPM for all speeds
for example a 2 gear Transmission vs 4 Gear Transmission
2 Gear Transmission from 10km/h to 50km/h
1st Gear
10km/h = 2000RPM
20km/h = 3000RPM
30km/h = 5000RPM -----> Shift to 2nd Gear
2nd Gear
30km/h = 2000RPM
40km/h = 3000RPM
50km/h = 5000RPM
now check here
4 Gear Transmission from 10km/h to 50km/h
1st Gear
10km/h = 2000RPM
20km/h = 3000RPM-----> Shift to 2nd Gear
2nd Gear
20km/h = 2000RPM
30km/h = 3000RPM-----> Shift to 3rd Gear
3rd Gear
30km/h = 2000RPM
40km/h = 3000RPM-----> Shift to 4th Gear
4th Gear
40km/h = 2000RPM
50km/h = 3000RPM
its a crude example but it makes it easier to understand
the 2 Gear Transmission keeps the Engine between 2000RPM and 5000RPM
but the 4 Gear one keeps the Engine between 2000RPM and 3000RPM
They can put in 9 Gears in a Manual transmission but they dont because it will be a headache as u will have to keep changing gears constantly
An Automatic transmission can do that easily without the aid of driver
its only limitation is the complication of manufacturing