It is necessary to format a disk because the disk must store data in a way that the computer reading it can recognize. There are many types of formats that can be used to store data in a computer. Which one that it is depends on what the device that stores the data is and what formats the disk. If a disk was not formatted, the computer would not be able to read what was on the disk because it would not recognize the data as anything usable (Its not practical to just have bits of data floating around, they must be organized). It can get deeper than that, but that is about why you need to.
Hope this helps!
Hope this helps!