Some of the lower ranking japanese that were serving the lord of there land in an official copasity but weren't samurai were not allowed to wear the katana, they used only a wakisazashi if they used a sword at all.
The wakizashi is used in many different styles that incorporate two swords, the most notable of which is Musashi's niten ryu. The wakizashi was often held in the left hand and exploited for its advatage in close-range and defensive capabilities.