1) My best guess would be that it's an image file created with dd of some sort of media like a flash drive or memory card.
2 and 3) Whether it's necessary or not depends on your definition of necessary. The operating system should run just fine without it, however it may contain data that you'd rather keep.
You can test my theory easily by using the command parted image.dd
which will result in output similar to but probably not identical to this.
Using /home/me/Desktop/image.dd
Welcome to GNU Parted! Type 'help' to view a list of commands.
(parted)
issuing the command print at the (parted) prompt will provide output similar to but definitely not identical to this
Model: (file)
Disk /home/me/Desktop/image.dd: 500GB
Sector size (logical/physical): 512B/512B
Partition Table: gpt
Number Start End Size File system Name Flags
1 1049kB 420MB 419MB ntfs Basic data partition hidden, diag
2 420MB 693MB 273MB fat32 EFI system partition boot
3 693MB 827MB 134MB Microsoft reserved partition msftres
4 827MB 470GB 469GB ntfs Basic data partition msftdata
5 470GB 470GB 891MB ntfs hidden, diag
6 470GB 471GB 472MB ntfs hidden, diag
7 471GB 500GB 29.2GB ntfs Basic data partition hidden, msftdata
(parted)
once you have your output enter quit
at the (parted) prompt to exit parted.
In your specific case, the image does not appear to have a partition table as shown above, but as it's an image of a storage device it has Sector Size info.
If you wish to peruse the content to insure you aren't deleting something you want, there are several answers here on how to mount an image as a loop device.
This is a fairly simple procedure and since there's no partition table you don't need an offset.
mount -o loop image.dd mountpoint
where mountpoint is the location on the tree where you wish to mount it. You may wish to create a mountpoint in advance.
For example:
mkdir /media/test
mount -o loop image.dd /media/test
ls /media/test
To make the directory
loop mount the image file
and list the content at the root of the image