That's the purpose of those rules: To give flight attendants a plausible safety-related task. That takes the passengers' minds off the fact that airliner safety rituals for passengers are absurd and that other aspects of in-flight service are sub-par.
Sure, they may be absurd, but the safety record of commercial air transport is absurd as well. (If you don't want or appreciate that safety level, light, ultralight or experimental aircraft are good options that allow for different risk tradeoffs).
