Sora's Keyblade is known as the "Keyblade of Worlds' Hearts", and nearly all of his Keychain tokens represent some important item from a world he has saved. Usually, Sora is not actually shown to be given the Keychain from one of his many friends, so it is possible that after saving a world, or completing a major step in that direction, the world itself rewards him with the ability to access its power in the form of a unique Keychain.
Keychains that Sora receives from his friends, or from defeating challengers in the Olympus Coliseum, always use a token that is somehow intimately symbolic of that person. While Sora's Keyblade is not specifically a Keyblade of human hearts, like the explicitly named Keyblade of People's Hearts, it is likely that the connections between his heart and those he meets allow him to sometimes access their power as well. Sora and Roxas's Oathkeeper and Oblivion are said to specifically embody Sora's memories of Kairi and Riku, and so it is extremely likely that for "people-Keyblades" like these, the Keychains act as a physical embodiment of the memories Sora has of those that are important to him.
Keychains that Sora receives from his friends, or from defeating challengers in the Olympus Coliseum, always use a token that is somehow intimately symbolic of that person. While Sora's Keyblade is not specifically a Keyblade of human hearts, like the explicitly named Keyblade of People's Hearts, it is likely that the connections between his heart and those he meets allow him to sometimes access their power as well. Sora and Roxas's Oathkeeper and Oblivion are said to specifically embody Sora's memories of Kairi and Riku, and so it is extremely likely that for "people-Keyblades" like these, the Keychains act as a physical embodiment of the memories Sora has of those that are important to him.