The computer uses unallocated space for what

Unallocated space is used as a new single partition.

The physical space on a hard drive that is not partitioned is considered as unallocated space.  

The programs use the space that is existing in the partitions. Space existing in the partition is termed as allocated space.

So, when there is no partition, the space cannot be used by the programs. This unallocated space is non-existent for the Operating system.

The space in the drive is partitioned during the system installation. In such cases where unallocated space exists, the system can create a new partition through the Disk Management control panel. Right-click on the unallocated space and left click on “New Simple Volume”. This turns all the unallocated space into a single new partition.

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