The speakers can draw more power from the amp. Drawing more power than the amplifier was designed for will damage the amp.
Every amplifier is designed to handle a certain load.
Most quality amplifiers can also handle a lower resistance. A good rule of thumb is not to go below 2 ohms or above 16 ohms. With stereo power amps, always avoid running 1 speaker cabinet one side and two or three on the other. This goes for any unbalanced number of speakers on one side. The ohms are bound to be messed up and you could damage that channel or the whole amp.