Final answer:
The IRA sniper must leave the rooftop in 'The Sniper' to avoid being targeted after revealing his location through gunfire, making his position vulnerable to enemy attacks.
Step-by-step explanation:
In the story The Sniper, it is necessary for the IRA sniper to leave the rooftop to evade the dangers associated with being discovered by the enemy. After engaging in a fierce battle and revealing his position, the sniper is compelled to find a safer hiding place to avoid being tracked and targeted again. The nature of his action—firing at an adversary—makes his location vulnerable to return fire or counter-attacks. Hence, relocating becomes an urgent matter of survival.