Why is assessment an ongoing process?

Why is assessment an ongoing process?
 
  a. Change in clients' lives, problems, and other developments require revised
  considerations of clients' situations and modifications in the work
  b. It can often times be such a difficult task to undertake that it needs to be
  done slowly and in bits and pieces here and there throughout the work
  c. Clients may change their minds in regard to certain circumstances, situations,
  and people in their lives
  d. It is not an ongoing process because the initial assessment should reflect the
  clients' situation accurately