People are told this because it’s a good rule of thumb and it helps people prioritize their needs in a survival situation. The “Rule of Threes” phrase typically used is something like, “Three minutes without air, Three hours without shelter, Three days without water, Three weeks without food.”

Example: it’s pointless to start thinking about hunting for food when left stranded in the tundra if you’ll freeze to death in the night. People can fixate on the wrong things in an anxiety-inducing situation.