Speaking is more important in early stages，then you build up your confidence,accumulate more words，simple ones，sophisticated ones,and you can reads more and more,sooner or later,there goes on good writing.

   Spoken English employs a much smaller vocabulary.

   Written communcations typically involve a much more extended vocabulary.

   More    credence is ceded to written communications.

    That is why  a  written contract  bears more force in law  than an oral contract.

   That which is spoken leaves no record,  generally,  unless recorded by some electronic device, which is a modern,  but not  an ancient custom.

   Also,  spoken language  is commonly usef for mundane activities.

However,  even in the use of spoken language,  when the subject  of dialogue becomes 

serious, rather than an exchange of pleasantries,  vocabulary  that is associated with abstract concepts is needed for communication. Thus, the vocabulary  commonly associated with written communications can be employed in speech.

   If   written communications  were  limited only to the vocabulary commonly used for daily speech communication,   there would be little to write.

   Thus,  when I practice with a student to enlarge their vocabulary, I rely upon something that is written that  I can e-mail them.  If I had only  spoken words to use for my lessons, 

I would  find practices in this lanugage very difficult.

I would say that to focus on only one skill is not a very good method for learning a language, ideally, you need to work on all four. This is why most coursebooks have activities for listening, reading, writing and speaking....