Why would a vSphere administrator configure resource shares for a virtual machine?

Shares establish a proportional priority for resource access specifically during contention - when VMs compete for CPU, memory, or storage I/O, the hypervisor allocates those resources proportionally based on each VM's share value relative to other VMs. Outside of contention, all VMs can consume resources freely regardless of their share settings.

Reservations - not shares - guarantee a minimum amount of a resource to a VM; shares only influence relative priority during contention, not guaranteed access.

Shares have no effect when resources are abundant; the prioritization mechanism only activates under contention, so shares cannot prioritize access at all times.

Guaranteeing resource access at all times requires a reservation configuration, not shares; shares are purely a relative weighting tool during resource contention.