Formatting is creating data storage medium, generally a hard disk drive, for reading and writing. During the time you format a disk drive, the OS wipes out all bookkeeping data on the disk, tests the disk drive to be sure all sectors are quality, marks bad sectors (i.e., those that are scratched), and makes internal computer address tables that it afterwards utilizes to find data. One must always format a disk before you can use it.
You generally seem to alter the file system & OS like we upgrade from Fat to Fat32 or Fat32 to NTFS. Or it can happen we downgrade our file system for backward computer software compatibility.
However this is the canonical reason but other than this formatting is nearly the solution from the entire day to day problem of an OS like slow start, virus attempt, system error, bad sector and several other problems. In reality formatting is the shortcut solution for every problem.
During formatting you will sometime format the incorrect drive or forget to take back up of crucial data files. Or by formatting your complete drive unintentionally, rather than formatting a single drive.
Remember that formatting or reformatting a disk does not delete the information on the disk, just the address tables. Do not worry, in case, if you unintentionally reformat a disk that has useful information. A computer savvy must be able to recover files from formatted drive, if not all, of the information on the disk. In addition you can also purchase Mac disk recovery software applications that allow you to retrieve a disk yourself.