On-Site Procurement

In several games, the player has to gain various upgrades (whether they are healing items, weapons, or other equipment) that are scattered throughout most of the game. Sometimes the item cannot actually be procured due to not being experienced yet, and 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.

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.