It's easy.... imagine a motor spinning at 10 turns / second... then reversing... it's now turning -10 turns/second.

When you have I and Q (Information and Quadrature of it), you can tell which way a signal is "spinning"... and there are many things you can't do if you don't have both parts of the signal.

If you have two antenna near each other, you can look at the phase and tell where the signal is coming from, in terms of which is closer to the source, by that rotation direction.