@Dougie is absolutely correct, you can to whatever you want with this memory, and it will survive a power cut of the main supply as long as the battery is OK.
The reason for such memory is that in microcontroller projects, there's often the need to store some data so that it survives a power cut. Many microcontrollers have builtin EEPROM, but EEPROM can only be written a limited number of times (order of 100.000 times). So, writing to EEPROM whenever the data changes might be a bad idea. Writing only in the event of a power cut might be OK, but adds complexity to detect and react to it. The microcontroller supply voltage must sustain long enough to write the data to EEPROM, and since EEPROM writes are quite slow, this needs big buffer capacitors.
RTCs are ultra low power devices, which are supplied from an external power supply, but switch to the battery on a power cut. They already have a serial interface, so adding some SRAM, which also needs almost no power to keep the data is a good idea. So, that's why you often find some RAM in RTCs.