Storage policies give you granular control over how the data for any given VM or VM disk is handled.
Storage policies allow you to define how many mirrors (RAID-1) and how many stripes (RAID-0) are associated with any given VM or VM disk.
Storage policies are applied at VM creation to begin with and can be changed later as the reliability or performance needs change.
The policies themselves can be modified if all associated VMs need to be changed in the same way.
If no policies are defined, VSAN will use its default policy to provide simple protection to any VMs created without explicitly defined policies.