Setting a multi drive unit to RAID 0 gives you access to the full storage space (ie. an 8TB unit will have approximately 8TB of available space). The negative of RAID 0 is that there is no backup/redundancy of any type. If one or more drives fail, all data will be lost.
Setting a multi drive unit to RAID 5 requires one drive for redundancy (ie. an 8TB unit will have approximately 6TB of available space). While available space is reduced, data is protected from a single drive failure.