This is to prevent overloading bridges and roadways. Overloading can have bad effects on steering, braking, and speed control. Overloaded trucks have to go very slow on upgrades. Worse, they may gain too much speed on downgrades. Stopping distance increases.
Brakes can fail when forced to work too hard.
During bad weather or in mountains, it may not be safe to operate at legal maximum weights. Take this into account before driving. The height of the vehicle's center of gravity is very important for safe handling. It is most dangerous in curves or if you have to swerve to avoid a hazard. It is very important to distribute the cargo so it is as low as possible.