Users make mistakes. If your program expects an integer and they type abc, the input operation fails. Without checking for this, your program uses garbage values or enters an infinite loop. Three common input errors:
Wrong type (letters when expecting numbers),
Out of range (typing 999999999999 for an int),
Empty input. Each leaves cin in a fail state. When cin fails, it stops accepting new input until you clear the error state and remove bad data from the buffer. Without fixing the problem, subsequent reads fail immediately.