Rats there are many types, but for this reason I will generalize. 
Rats teeth grow like our nails grow so they must gnaw at hard objects to stop them from growing. Lately there has been an increase in this behaviour because the plastic coating around wires and plastic components in car used to have an oil base, in recent years the manufacturers decided to go Eco-friendly and make the wiring coating with a soy / corn base so they would be more bio-degradable, but unfortunately rats love soy and corn hence them eating the wires and causing thousands of Rands worth of damage.
Rodents also carry diseases such as salmonella gastro-enteritis and numerous other diseases.
They breed at an alarming rate, it is estimated that a pair of rats could grow to 2000 within 1 year.