ahemPuts on his Musashi fanboy hat.Musashi began working on the two sword style after he retired from dueling, after approximately 60 undefeated duels. He challenged the mindset of not using the wakazashi, which was primarily a ceremonial blade representing the Samurai class. He tested and spoke of using many different techniques, such as using the short sword in your primary hand as it was more nimble and accurate while the off-hand could use the larger sword to protect a wider area; as well as the more common practice of using the larger sword in the primary hand for more powerful strikes. However it was more important to Musashi to be prepared and skilled for any situation, which is part of why he advocated training with the off-hand as well as two full swords.
While he never used the Niten Ichi style during his duels (because he hadn't started it's development yet), it is said that on occasion he would take a second sword from a fallen foe and use two swords when he was fighting against a gang who were pissed that he was beating them up. As Jorundr said, it's because it's idiotic not to use any weapon of any opportunity.
Jorundr is also correct in that Musashi whittled a bokken out of an oar and
**** his opponent's head, Sasaki Kojiro, after a number of challenges in which Musashi used psychology to unnearve Kojiro.
The Go Rin No Sho, the Book of Five Rings, is less about actual fighting but about the philosophy of a warrior, and how it can be applied in different situations, for Musashi not just in personal fighting but also for tactical warfare and for embracing other arts, including poetry and ink art. In this aspect it's similar to Bruce Lee's Jeet Kun Do philosophy.