Many devices that are used with the RPi / Arduinos are powered by 5 volts and output 5 volts, BUT the microcomputers' inputs are often limited to around 3.3 volts. Voltage dividers are inexpensive, fast, and easy ways to bring down 5 volts to a usable 3 volts.
WARNING: Miswiring your microcomputer can ruin it and any attached device. Information offered "as is". You are solely responsible for how it is used.