Why? For example, you may want GND/PWR separate CLK signal from the others, it helps minimize crosstalk between signals on a cable. You may want to consider how currents will flow. You may want to prevent board damage when the mating connector is inserted the other way round.
Note: This is a tip from my new book which I am preparing. If you would like to be notified when this book is ready, just fill out this form >
Welldone Blog 