Parallel can be better for simple high speed/critical timing applications, like pulsing a motor driver to hold a controlled velocity. You can turn a pin on or off in one operation, so it's quick. Since the pins are hardwired to specific control circuitry, it's very fast saying to what thing you're changing the binary value.
Serial (on a stock PC) is great if you have things that you want your PC to talk to on a slower pace. The data you send/receive can be simple (like a data byte) or quite complex like a string of characters. Things like: a simple DRO, a tachometer, or a keypad would probably be good applications for serial, as long as it's not too performance critical.
Serial has to clock in (pulse a clock signal after setting the data line on or off,) some minimum number of bits to do anything really useful, so you have to wait for at least 8 clock cycles (115k/second [or higher]) to write out a byte. Since it took some thought to explain that, you can understand that the complexity is a lot more than parallel. (USB is even more complicated.)
So, it depends what you're doing. Motor drivers call out for parallel - cheap, fast, easy.  |