I personally use them with the alarm function of an RTC.
The processor can completely ignore the clock until such time as it receives an interrupt. Then it can divert and go and do whatever it needs to do at that time. Useful if you have the processor in sleep mode. Mr Gammon has an excellent piece on this subject on his site, under power saving.
I would also point out that he was the first person to try to help you. If you don't understand the answers then either Google it or ask about the part you don't understand. Nobody minds helping someone who is trying to help themselves.