Anchor is a Roblox Studio tool and a property of BaseParts that allows a BasePart to be static from the physics engine, and will remain in that position even in edit mode. Any physical events that happen around the BasePart will not effect itself until unanchored.
To anchor something, select the Anchor tool, then click the part to be anchored. To unanchor, click it again. To do this with the Properties panel, check or uncheck the Anchored property.
Note that anchored parts will not move, and anchoring tools will cause the player to act anchored until they unequip the tool.
It is used for Baseplate in many games to protect Baseplate from falling to the void, preventing the game to become unplayable.
It is used in many houses and buildings to prevent parts from collapsing.
It prevents parts from obbies to fall, preventing the game to be unplayable.
All though, some people like to make buildings or items collapse with this tool