SECONDARY STORAGE!

Secondary storage, sometimes called as external memory and auxiliary storage, a secondary storage device is any non-volatile medium that holds data until it is deleted or overwritten. Secondary storage is about two orders of magnitude cheaper than primary storage. Consequently, had drives (a prime example of secondary storage) are the go-to solution for nearly all data kept on today's computers.

The differences being that offline storage is used as a way to physically transport information, generally has less capacity, and isn't accessible without human interaction.
Typically, secondary storage is used to back up primary storage, which involves holding a secondary copy of the data. To choose an effective secondary storage media, one must first understand the data's value, how often it's used and how it will be accessed. External hard drives, portable USB flash drives, CD's,,memory cards and cloud backup are often used for secondary storage. Generally, secondary storage devices perform at a lower level than primary storage and are less expensive.
 We need secondary storage device for many purposes for example, in past(not going so far just few years back) storage of computers was less and if people wanted to store more data they had to use cds or other secondary storage devices. And also when we want to backup our data or we cannot carry whole pc with us we simply put up the required data in secondary storage and can also share with those who don't have internet facility. So collectively we can say that their are many advantages and purposes of secondary storage devices which are very useful.