I think it may be a fair guess Tsunade learned that seal from her grandmother who helped train her.
- You don't that seal to do the chakra enhanced strength jsutu which Sakrua has been using since the time skip. Tsunade herself probably did the same when she was younger.
- The big difference is with the seal the amount of power pushed into the attack is increased exponentially.
Also I wonder if the Kyuubi chakra Naruto gave her helped fill the seal.