Jujitsu is a form of martial arts that defeats opponents with various chokes and holds that incapacitate the opponent due to injury, pain, or lack of oxygen. However, Jujitsu also involves techniques for getting into these positions of victory. You cannot expect an opponent to let you get him into a chokehold, especially if they want to stand up and strike with you. Jujitsu throws and grapples are used to help the Jujitsu practitioner get the fight into the situation that he wants it to be in.
When two people are striking it is common for them to get into a clinch. In this clinch an experienced fighter will be able to effectively strike and grapple while an inexperienced fighter will simply want to lean and take a break. Jujitsu trains fighters to use the clinch to take the fight from a standing striking battle into a position where a joint lock or choke can be applied. While in the clinch a Jujitsu fighter will be able to land quick strikes, grapple with the opponent and control their movements, and then take them down using a throw.
Even when fighting a much larger person, you can use Jujitsu grappling techniques to control their actions by using leverage and angles. This prevents them from stepping away to begin striking you again, and allows you to maneuver them into a position in which you can apply a choke or joint lock. Effective grappling is based on using your arms to control an opponent due to their position, not their strength. Your arms do not need to be very strong to lock an opponent into a position and keep them from striking you. Using under hooks, which means having your arms locked under the opponent’s armpits, allows you to prevent them from striking and use your whole body to move them around.
Jujitsu also makes use of throws to get an opponent onto the ground where submissions are easier to achieve. Unlike wrestling throws that are accomplished by picking the opponent up using pure strength, Jujitsu throws use leverage and gravity to bring the opponent down. Usually this is done by taking them legs out from under them using a sweep or by collapsing their knee.
Jujitsu is so effective because the Jujitsu throws and grapples can be accomplished even by small martial artists. These techniques combined with the Jujitsu submissions allows skilled fighters to win against strong fighters on the merit of their moves and techniques. In many ways, this is the heart of martial arts.