Why do soccer players touch the field as they go onto it?

This question does not contain enough information to give a factual answer because the angle and direction of the referee signal would need to be provided for a definite answer. This is speculation:

A goal kick is awarded when the ball crosses the goal line outside the bounds of the goal. The referee points to the goal area to signal a goal kick is to be the restart. No further signal is necessary.

You may be confusing a goal kick with an indirect free kick. On an indirect free kick the referee's arm is held straight up during the kick.