Practically all television was broadcast live in the early days. Much of it originated from New York. After television began recording programs for later broadcast, production shifted to Los Angeles and live productions became a thing of the past.
In the 1990's, however, the television networks saw live broadcasts as a way to inject interest in shows that were falling in the ratings or get bigger ratings for successful shows. Fox chose to air live episodes of its underrated series Roc, which got a boost from the fact that its cast was used to performing in front of live audiences on Broadway.
ABC saw a ratings bump from doing live episodes of The Drew Carey Show. Taking advantage of its cast's standup and improv experience, the live episodes brought larger audiences to the show.
No network, however, has gotten as much mileage out of live stunt broadcasts as NBC. The network has gone to that well time and time again, using the gimmick on ER:
The West Wing:
And most recently 30 Rock:
