Flights are usually overbooked by around 5-10 passengers a flight, and is well-calculated based on the expected number of no-shows.* Depending on the ticket price, this can mean somewhere between $1-5k a flight, which can be a pretty substantial difference for a lower margin flight.

No, there's no benefit to deliberately overbooking to fill a later flight given that bumped passengers are compensated. At best they're giving away a seat on that later flight for ~free, and at worst they're eating the cost of another paying passenger that can't book that seat anymore.

*source: a friend working in revenue mgmt at United