Top of climb

TOC establishes a "starting gate" for the en route portion of a flight to allow timing and tracking along the course.

Once past TOC (and by definition stabilized on course) a pilot can efficiently ascertain whether the wind and weather are "as forecast" and the groundspeed attained will allow for the safe completion of the flight "as planned" or within an acceptable margin for safety.

Unlike when driving a motor vehicle, the aircraft's progress over the earth (groundspeed) is in every instance unique and surprising.

Despite planning from forecast data, it is essential to immediately determine this in flight and adjust for variability to assure a safe outcome.

The airplane's Pilot Operating Handbook has a table of fuel burned, time, and distance to reach a given altitude from sea level.