Question about our use of the Arduino/ATMega328


I'm a mechanic, so I have a general idea of how the electronics work, but I'm fuzzy on the details. Could someone explain to me simply what we use the ATMega chip for on the rov cape? Is it just because we need 5V control signals that the Beaglebone can't provide and using one ATMega chip is easier than using a bunch of MOSFETs?