If you purchased a pair of speakers in the past these will be 2 matched mono speakers, which when connected to an amplifier would create full stereo sound. The idea is the same with in ceiling speakers, where you will need to purchase 2 speakers to have the same effect and high quality stereo sound.
Mono speakers on there own can only play the sound from one channel of audio (either the left or right channel). This is why it is not recommended to only use one mono speaker. To have the best quality audio depth for your home, it is recommended to purchase a pair of mono speakers.