Volatility of memory:

Non-volatile memory will received the stored information even if it is not continually supplied with electric power. It is appropriate for long-term storage of information. Today used for most of tertiary, secondary and off-line storage. In the year 1950s and 1960s, it was also used for primary storage in the form of magnetic core memory.

Volatile memory needs constant power to keep the stored information. Fastest memory technologies of nowadays are volatile memory (not a universal rule). Since primary storage is needed to be very fast, it mainly uses volatile memory.