The two halves of the system
A mattress encasement and a pillow protector do different jobs. The mattress cover protects the expensive asset underneath. The pillow cover protects the thing your face is pressed into for eight hours a night.
Most properties buy the first and skip the second, then replace pillows three times more often than they need to. Pillows are cheap individually and expensive in aggregate, and a protector costs a fraction of the pillow it saves.