Writing is a Process
Writing is a complex process that requires the author to be aware of and combine various components of language successfully.
While the physical act of writing is fairly automatic for adult writers, in the L2 it becomes a conscious process once more, especially if the L2 orthography is different from the learners' L1. The same is true if the rhetorical style of the L2 is vastly different from that of the L1 (this is particularly relevant for longer writing assignments).
L2 writers spend less time planning and organizing ideas and have more difficulties with these steps (Silva, 1993). To counter this, L2 instruction should include time for planning both content and form, for generating ideas as well as for improving accuracy.