[kevin_thibedeau]

It used to be much more common. In the 70s there was a lot of collective hesitance to use hex with its strange letter digits. Octal was the compact representation of choice.

[adrianmonk]

Also, some very old computers had 36-bit words. Word sizes on modern computers are virtually always powers of 2, but it hasn't always been that way.

And octal is more convenient for output via 7-segment LEDs and for input via numeric keypads.