On-Site Procurement

In many games, the player ventures into danger with only what he's carrying, and has no supply line. Improbably, he has to acquire needed materiel and upgrades (whether they are healing items, weapons, or other equipment) from the hostile territory he's advancing through. Just why the enemy -- or Mother Nature -- has seen fit to scatter ammo boxes, first aid kits, and bigger guns than the player already has at random everywhere on the map (instead of, say, gathering them together in armories or infirmaries) is a question best not asked. Sometimes an item cannot actually be procured because the player isn't experienced enough yet to unlock them, requiring a backtrack to get it later.

Although several of the weapons and equipment are required for the plot to advance, there are also things such as ammo upgrades or health bar upgrades that are not actually necessary for anything other than a 100% completion.

The name for the trope comes from the Metal Gear series, where the action pertaining to getting most of these weapons and equipment upgrades was referred to as Procurement On Site. It's an easy justification for the hero getting sent out to save the world With This Herring.

A variety of Easy Logistics, although how easy is actually is varies with the game.

Despite how silly it is when viewed outside of the context of the game, this is an Acceptable Break From Reality. See also With This Herring.

Action

 * The Metroid Series has Samus trying to procure upgrades before attempting to fight the Big Bad on the Hostile Planet.
 * Averted with Other M, as Samus has to have her abilities unlocked by Adam, which also acted as a very controversial aspect of the game to some fans, especially the Hell Run portion where the Varia Suit wasn't verified by Adam until around the time she has to fight a lava monster boss.
 * In order to advance through the dungeons in The Legend of Zelda series, and ultimately for the game, Link has to procure weapons that more often than not also act as the boss's weakness.
 * As noted above, the Metal Gear series is the trope namer, as Snake has to procure enemy weapons on the enemy base/enemy territory as part of a sneaking mission. It's best justified in Metal Gear Solid, since Snake infiltrates the base in a submersible capsule with nothing on him but a pack of cigarettes (which even that was actually due to Snake smuggling the smokes in his stomach and not due to actually gaining permission to do so), and in Metal Gear Solid 3, where it's explicitly laid out that the U.S. government can't risk Naked Snake being captured on Soviet soil with American-made equipment and weapons.