1. As the TAF and METAR always give wind direction in degrees true, the variation is needed to convert this into a magnetic value to calculate cross-wind components relative to the (magnetic) runway directions.
2. ATIS broadcasts and 'tower' winds given over the Radio are in degrees magnetic, but you wouldn't normally hear these until you were in the cockpit, so the variation is supplied for pre-flight planning purposes.
Other than that, I agree that a knowledge of the variation plays little part in take-off, landing and ground
operations; it's more of an aid to
planning.
Hope this helps
Eck