It happens. Typically the defendant is in custody and usually only sees their attorney when transported to court, and usually only for short periods of time. If it’s a very simple case, a written statement usually isn’t necessary. But for more complicated cases, or cases where there’s a lengthy history or back-story that provided motivation for the crime, i.e. a lot of family drama, documents, accounts, or potential witnesses, it’s not unusual for counsel to tell the defendant to write down the facts and provide it to counsel the next time they’re in court together. Although not ideal, the reason some attorneys will do this is because they need an account of the facts, and since attorneys usually have more than one client to represent in court, (if counsel is a public defender, they always have multiple clients, and if counsel is retained, they try to schedule multiple appearances in that courthouse on the same day) visits at the courthouse usually don't provide enough time to obtain all the information they need. So it's either have the defendant provide a written statement, or counsel or counsel’s investigator has to make a trip out to the jail to visit and interview the defendant, which can be a lengthy distance away, and usually at night after a long day at work.
Having the defendant write a statement of what happened also has the added benefit that it helps the defendant remember what happened. Sometimes it takes years for a case to go to trial. The defendant's written statement is helpful to refresh their memory of important details the closer they get to trial.
Also, written statements aren't only provided when the defendant is in custody. I used to work in a public defender’s office that would give defendants who bailed out of custody an interview packet that included a section to describe the reason for their arrest, and defendants would bring it with them to their first pretrial appointment.
