I doubt they really expected to close on March 17 and make a profit. If it flopped, they would close on 3/17 without recouping; if they were a hit (which they are,) the extensions were designed in advance to allow them to turn a nice profit. It does create buzz when you announce that you're extending. Twice.
Announcing an end date creates a necessity to act and get tickets. That pushes the audience into a certain number of weeks. Once that urgency is created, the extensions - as it was pointed out - generate positive buzz and keep the show in the public's eye.
