When an application is directly powered from batteries the operating voltage of the MCU and the I/O voltage level drops as the battery voltage drops. This may cause issues with interfacing to other electronics and inaccuracy is analog functions. The voltage of a typical battery cell ranges from 1.6V when fully charged down to 0.8V, dropping roughly 50%. On the other hand, the operating voltage and I/O pin voltage levels of ATtiny43U will stay constant during the whole battery life, down to 0.7V. The level stays high enough to effectively run the micro and the I/Os.