A formal witness statement can be put in , as though the witness was present in court, if the parties agree. It is as good as any other evidence. It is just that the defence have no questions to ask of the person who made it and they accept it as true. There is no point in wasting the person's time in making them come to court for the trial, if that is the case.
All other statements, if relevant, are made by witnesses who will attend the trial and give live evidence. When the statement was made goes to how accurate the person's recollection is. It is common for the defence to put the statement to the witness, to show that what they are now saying in court differs from it or that what they can now not recall was recalled by them in the statement. The nearer the statement was to the events, the more accurate it is likely to be.