Difference between using voltage control and percent control

I noticed that most of programmers use voltage instead of percent to control their robots when I watch videos about pid on the internet.I used to use percent to control my robot, but now I use voltage. When I use voltage to control my robot, I find that my robot moves more smoothly.Why does using voltage control have this effect?

Simple: Percent has brain altering current automatically to meet the percent, voltage has current not altered by the brain

Kinda technical:
Because using percent enables a built-in pid loop to meet the percent and stay near it while the voltage control interacts directly with the motor and send a steady flow of electricity. The built-in pid is usually not tuned to the bot as well as a user-made one would be, which can result in oscillating and nonsteady speeds. This can be noticable in flywheels as the oscillation when the percent is less than 100.


I must also note: I have not dug into any source files, this is just an educated guess based off repeated observations