When should we use memory engine?

I am newbie to MySQL and don't know much about MySQL storage engines.  Can you explain, when should we use Memory Engine?

Among others, you might consider using a MEMORY table when your data is:

• Negligible: The target data is relatively small in size and accessed very frequently. Remember that storing data in memory prevents that memory from being used for otherpurposes. Note that you can control the size of MEMORY tables with the parameter max_heap_table_size. This parameter acts as a resource safeguard, placing a maximumlimit on the size of a MEMORY table.
• Transient: The target data is only temporarily required, and during its lifetime must be made immediately available.
• Relatively inconsequential: The sudden loss of data stored in MEMORY tables would not have any substantial negative effect on application services, and certainly should nothave a long-term impact on data integrity.