RAID 5 setups allow for getting the most storage out of an array of disks without loosing parity, or redundancy. For example.
Raid 5 array with four disks will have the capacity of three of those disks using the forth's pool of storage (over all the disks) as a parity bit so if one of the disks were to fail you would simply swap out the defective disk and the array will rebuild itself in an ideal world.
Because of this write speeds on RAID 5 arrays tend to be a bit slower then other arrays because it's spreading out it's data on however many disks you have any it's writing parity bits at the same time. Read speeds tend to be quick on RAID 5 making it ideal for archival use or data that must be read a lot but not edited much or changed. Think of a server dishing up programs to users or storing their finished work files.
Hope that helps!