Structural linguistics claims that spoken language is primary and written language secondary, while sociolinguistics views them as two distinct entities that influence each other in complex ways. From the perspective of communication - and as far as I am concerned - speaking and writing complement each other.
The typical form of speaking, excluding prepared speeches, is a spontaneous and therefore synchronous mode of communication, while the typical form of writing, excluding instant messaging, is a contemplative and therefore asynchronous mode. This means, again as far as I am concerned, that if I communicate with someone only through speech, I encounter only one mode of their communication, while the other remains hidden.
Though I love spontaneous spoken interactions, I value written dialogues no less, especially when corresponding with someone who can express spontaneous thoughts and feelings at length and in a meaningful way that naturally elicits my response.
What I find equally important about such written dialogues is that they require us, first and foremost, to engage in inner dialogues. Contemplating our own thoughts and feelings and formulating them in writing makes these thoughts and feelings clearer not only to others but also to ourselves.
As a result, I have come to realize that many people who lack this habit tend to express their thoughts and feelings less clearly even in speaking than those who cultivate it. This is also why, in retrospect, almost all the meaningful relationships I have had - romantic or otherwise - have been grounded in such written dialogues, allowing me to connect deeply with others through two complementary modes of communication.