Complex sentence: It is marketed as a target designator that is useful for long range target practice: the shooter does not need to walk down-range to see if the target has been hit, as the Tannerite will detonate and serve as a highly-visible indicator.
Split into simpler sentences: It is marketed as a target designator that is useful for persons who are firing at long ranges. The shooter does not need to walk down-range to see if the target has been hit, as the Tannerite will detonate and serve as a highly-visible indicator.