By blessing God 100 times daily, we are connecting in the deepest way. Mindfulness of our actions makes us more aware of the Divine. It also serves to prevail over the forces of negativity, by attaching ourselves more closely with Hashem (God).
King David instituted 100 blessings when there was a plague in Jerusalem killing 100 people a day. After people began saying 100 blessings, the plague subsided. The plague, as explained by Kaballah, is representative of the sitra achra, the "other side" or in our terms "the dark side." The power of each blessing, which invokes the four-letter name of Hashem, protects against the power of the the sitra achra, and draws us into a closer connection with Hashem.
Below is a class based primarily on the work of the Ben Ish Chai discussing this in greater detail and also outlining the way to achieve saying 100 blessings a day according to Jewish tradition.