What is the primary reason an alternate airport is required for an arrival?

The primary reason an alternate airport is required for an arrival is that it ensures weather conditions at the destination are above specified minimums. This requirement is crucial for flight safety. In the event that the weather at the intended destination deteriorates unexpectedly or is below the minimum visibility or ceiling requirements for landing, having an alternate airport allows pilots to divert and land safely elsewhere. Designating an alternate airport acts as a safety net, ensuring that pilots have a viable option if conditions change, thereby reducing the risk associated with landing at an airport where weather conditions could impede a safe arrival.

While fuel efficiency, runway options, and time considerations are important aspects of flight planning, they do not take precedence over the fundamental need for safety provided by meeting the weather criteria at the destination.