Where are you looking in Computer or in Disk Management? Is it actually 2 physical drives or is it one drive partitioned? If it is actually 2 physical drives there a number of reasons. Automatic backup, extra storage, run two OSs one on each drive.
What is the size of the third partition Q ? It may be a system restore image or if it's small like 200mb it may be boot files if its 10-15Gig it's a system restore image. Can you get a shot in Disk Management? Get the portion with the bars.
Looking at the size of the drives I would guess that this is one 500 Gig hard drive partitioned into two drives.
And that the 3rd partition is the recovery partition.
The reason for doing this is generally so that if you have a failure of Windows or a virus of some sort that requires re-installing the operating system completely you can do so without losing all you files.
With the hard drive partitioned you can format drive C:\ and leave everything on Drive D:\ untouched.
I always did this in the old days when I had only one hard drive.
Now I always have multiple drives and I don't have anything except software and basic Widows stuff on my C:\ drive.
I have that all backed up to another drive so it's not worth partitioning anymore.