Knowing how and what your duo is going to be looking for is pretty important. Especially if in solo queue. Plus you then get to have both the knowledge and experience of how and in what way's to best help each other. Trust me, it really helps your ADC game, as well as offer you a good option for when you are auto-filled. Since both are in the bottom lane, and both are duos for each other, your laning knowledge is fairly similar and easily applicable to each role.