When an F1 driver keeps their right foot on the throttle and used the left foot for braking, it is called left foot braking. This style of driving was made popular in the 1990s due to the invention of hand clutches.
This driving style comes in handy when cornering or cruising at high speed. This type of driving is common with NASCAR drivers as well as Rally drivers. The main purpose of left-foot braking is to reduce braking time.
Another reason this left-foot braking became popular in F1 is the lack of a third pedal. The footwell of an F1 car is quite narrow and so this method seems to be more efficient. It provides comfort when driving a car with a narrow pedal box.