The study of leaner language helps teachers to assess teaching procedures in the light of what they can reasonably expect to accomplish in the classroom.
It also helps learners to be aware of the steps that they go through in acquiring L2 features.
It provides a deeper understanding of errors that L2 learners make. An increase in error may not result from a lack of practice or transfer from L1; rather, it can be an indication of progress (e.g., due to overgeneralization).