Someone needs to go on an adventure or otherwise leave their usual routine behind. However, whether it be because of a Secret Identity, the Masquerade, or whatever, nobody involved in that person's normal life can know that they're gone. Thus, the Substitute Self is needed: a robot double, a hologram, or just someone else in disguise to take a character's place. Related to Identity Impersonator, which is a temporary form of this done specifically to protect the Secret Identity by having the substitute and the main character seen together. Sleeping Dummy is a version of this for sleeping, which can be much less intricate because it doesn't have to act like the person.
Hello, Unknown Troper. You'll need to get known to lend a hand here.