The trigger is the enemy ending its turn nearby

An enemy merely moving next to you does not instantly trigger Skirmisher. The timing occurs when that enemy's turn ends within the required distance.

It uses your reaction

If you already used Uncanny Dodge, made an opportunity attack, or spent the reaction on another feature, Skirmisher is unavailable until the reaction refreshes.

The movement is protected

The movement granted by Skirmisher does not provoke opportunity attacks, allowing a ranged Rogue to create space without spending the next turn's bonus action on Disengage.

Half speed scales well

Superior Mobility later increases the Scout's speed, which also improves the distance that half-speed reaction movement can cover.

Do not retreat automatically

Sometimes holding position is strategically better. Use Skirmisher when the new position improves safety, line of sight, target access or the next turn's Sneak Attack opportunity.