Water mitigation
Mitigation is not repair. It is the work of stopping the damage where it is — before anything is rebuilt, and often before anyone knows the full extent.
What the equipment is actually doing
Air movers do not dry material directly. They break the boundary layer of saturated air sitting against a wet surface so evaporation can continue. Dehumidifiers then pull that evaporated moisture out of the room air. One without the other does very little, which is why both run together.
Readings are taken daily against a dry reference elsewhere in the house. The equipment stays until the numbers match, not until a set number of days has passed.
Containment and why the plastic goes up
Sealing the work area keeps humidity concentrated where the drying equipment is, and keeps dust and spores from being distributed through the rest of the house by the air movers. In a house with forced-air heating, the ductwork is usually closed off in the affected area for the same reason.
The part that gets skipped
Cavities. A wall that is wet inside will not dry through the drywall in any reasonable time. Small holes at the base of the wall, removal of the bottom few inches of drywall, or drilling behind the baseboard lets air reach the cavity. A job that dries the surfaces and leaves the cavities is the most common way water damage comes back as a smell three months later.