Why does every show start off with previously?

[Amanda Hugnkiss]

Supergirl does it, and Black Lightning does it, and MacGyver does it even though the last episodes weren't exactly cliff hangers. Black Lightning was awesome as usual, and the good news about that parson who likes to use profanity when he's behind the pulpit is the actor is just fine inspite of the fact that he played someone who took a gun shot wound that left the character paralyzed.