F1 has been improving the sport's safety for a long time. This is why wheel tethers were introduced in the 1988 season and are now mandated. Wheel tether is a safety equipment in the car, however, not entirely related to the driver's safety, but to others on the track.
Throughout F1's history, many drastic incidents have occurred on the track during a race or any other session. While crashes are devastating, many safety instruments are now installed in cars to protect the driver, however, there is another aspect that needs to be looked at.
During high-speed crashes, there is a high probability that a car might lose its wheel, which can be catastrophic for others on the track and turn into another crash for a different driver. This is why wheel tethers' use has been mandated.
These are nylon fibers connected to the wheel on one side and to the chassis on the other; since they are strong enough, they attach the wheel to the chassis and during a high-speed incident, make sure that the wheel does not fly around as a hazard for other drivers on the track. Its importance can be seen during this still from the 2015 Monaco Grand Prix:
Why are tethers in F1 cars not usually visible?
Though wheel tethers are an essential safety part of F1 cars, they are hardly ever seen while the cars are going through the circuit. The only time they can be seen by spectators is during a high-speed crash when the wheels break apart. This is because they are hidden in the suspension arms, so it becomes hard to be seen.
Initially, only a single-wheel tether was used in the cars, and although this stopped the wheel from flying around, it was very much possible that the rest of the wheel assembly, which includes the rim, tire, and brakes, could fly off. Because of this, the FIA and other authorities have mandated the use of two wheel tethers for every wheel.
Even with the installation of all the safety equipment, car parts, sometimes, detach causing a hazard to others on track. While other drivers are mostly protected because of the installation of the halo on their cars, it is still likely to cause a crash.
F1 is continuously working on making the sport even safer. This also helps other motorsport series to increase their level of safety, as can be seen from the adaptation of the wheel tethers and the halo by most recognized motorsports around the world.