More and more devices run on 3.3V or even less. Some have 5V tolerant data pins, but others can be damaged with 5V. The SN74LVC245A 8 Bit Logic Level Converter fixes that problem.
The SN74LVC245A is an 8 bit bus transceiver with 3 state outputs. It has 2 buses (A & B), both 8 bits wide. It can transmit from A to B or vice versa, depending on the state of the DIR pin. The input side is 5.5V tolerant and the output side matches VCC when high, so when interfacing with a 3.3V device, set VCC to 3.3V. The datasheet has a lot more detail.