Small unallocated space left, when partitioning harddrive. What is is meant for and why can’t I use it

There may be two main reasons why partitioning isn't a well supported feature on optical media:

• Optical media was at the beginning small (in comparison with HDD's).
• Optical media was read-only. This later evolved due to easier access to consumer-level disk burners and the advent of RW technology.

Nowadays, there are much better solutions such as pen drives or even portable HDD. Not everyone has a Blu-Ray reader and not everyone wants to purchase a RW DVD. As such, in what regards optical media, partitioning was never demanded enough for it to happen.

I do have to point out the answer Rod Smith gave to a similar question, which I'm now quoting:

There are exceptions to this rule, though. Some Mac discs are written with a partition table and HFS or HFS+ instead of or in addition to ISO-9660. Such discs use the Apple Partition Map (APM) partition table. Some Linux distributions' installer discs use ISO-9660 that coexists with a partition table, but the intent here is to support different media in differnt ways -- when the image is burned to a CD-R or DVD, it appears to be a partitionless ISO-9660 disc; but when it's written to a USB flash drive, it appears to be a partitioned hard disk. This trickery works because of gaps in various data structures and the different sector sizes used on optical media (2048 bytes) vs. hard disks and USB flash drives (512 bytes). Both of these examples (Mac discs and Linux installation discs) are hackish extensions to the standards of ISO-9660, MBR partitions, and so on.

As you can see, it isn't technically impossible to do, but where it's done it's to provide additional support that couldn't be done in another way.

The secondary label at the end of the volume is required by GPT; a disk without it is corrupted and may not be recognized by all systems. Using a corrupted format in order to gain a tiny amount of space, on a disk that's probably a few hundred gigabytes at least, doesn't make much sense.

However, the actual GPT label is only about 68 kilobytes in size. Modern partitioning software typically aligns the start of each partition on a 1MB boundary for performance reasons, since that's a multiple of any realistic disk sector or flash erase-block size, but there's no real need for the end of the last partition to be on a 1MB boundary. GParted may do that by default, but gdisk lets you create a partition that goes right up to the last usable sector (68k from the end of the disk).

Note, however, that the 1MB alignment of the beginning of your partitions means that you also have about 1MB of unused space at the beginning of the disk, between the GPT label and the first partition. If you know your disk's physical sector or erase-block size, you could align your partitions to that instead of using 1MB boundaries, in order to move the beginning of the first partition closer to the beginning of the disk. (But this seems like more trouble than it's worth.)