What is the merit and demerit of Right hand driving and left hand driving?

Question

in some counties they are using right hand driving but in some other nations they are using left hand driving why they are used so

Answer 1

About 34% of the world by country population drives on the left, and 66% keeps right. By roadway miles, about 28% drive on the left, and 72% on the right

AdvantagesLeft Hand Driving

Advantage of Right Hand Driving

When driving a car, a right-handed driver can operate the gear shift and other dashboard controls requiring fine motor skills with their right hand, while using their left to operate the steering wheel, which requires larger, less fine, movements. Some prototype race cars, even in countries that drive on the left, are usually configured with the gear shift on the right although it is unclear what the background behind this decision is. The FIA, motor racings governing body is based in France, a left-hand drive country.
For bicycle and motorcycle riders, most arm signals are done with the left arm, allowing the right arm to steer the bike, which is easier and safer for the majority of people who are right-handed. This is especially important since handlebars have a steering ratio of 1°:1°, unlike a car, which is often closer to 10°:1° (10 degrees turning on the steering wheel results in a 1 degree turn of the wheels).
Approximately two-thirds of the world's population, and more of the driving population, live in countries that drive on the right. With increased international travel, it is safer and more practical for a country to drive on the same side of the road as its neighbours. This is the most common reason for countries to switch to driving on the right.
It is easier to go in reverse when driving on the right. This is due to the body's natural torque of using opposing left foot and right arm for support that cannot be used when driving on the left since the right foot is on the gas or brake pedal.

With a right-hand-drive car, given that most people are right-handed, the less coordinated hand is used for changing gear and operating dashboard controls, leaving the more coordinated right hand free to steer. Driving on the left avoids the difficult combination of steering with the left hand and changing gear with the right hand and at the same time viewing the oncoming traffic with the left eye
It is more common to be right-eye dominant. Traffic flows in a clockwise direction when driving on the left which enables right eyed people to use the right eye to see oncoming traffic. When overtaking on a right-hand-driving road the right-eyed driver looks in the outside mirror with the left eye and also views the oncoming traffic with the left eye which is not suited to the majority right-eyed people.
When reversing and looking over one's shoulder, the driver is able to keep the more coordinated right hand on the steering wheel in a right hand drive car. This also enables easier viewing through the rear window.
When driving on the left, right-handed people mount bicycles from the kerb, who find it easier to put their right leg over the bicycle. This keeps a right-handed bicycle rider out of the stream of traffic.
Research in 1969 by J.J. Leeming showed that countries that drove on the left had a lower accident rate than countries that drove on the right, but Leeming's research is sharply criticized in Peter Kincaid's book on the rule of the road. Some countries that have switched to driving on the right (for example Sweden) saw their long term accident rates increase by more than any increase in traffic volumes.[citation needed] It has been suggested, but not proven, that this is partly because most people are right-eyed, and are therefore better able to judge the position of oncoming traffic when they see it on their right.