Answer
Many cases are listed for trial at a Crown Court without a court room and it means the witnesses may have to wait around at the Court all day for their case to be allocated to a court room.
To be on standby at court means that you do not have to be at the court all day waiting for your case to be allocated a court room. If agreed, you may be able to stay at work/home until your case is ready to be heard. Although you must not be too far away and must be contactable by phone and ready to go the court straight away if necessary.
You would need to check with the Witness Liaison department (the name of this department may vary from force to force) as to whether you could be placed on standby. If you are contactable by phone and not too far away from the court then there should be no reason why you could not be placed on standby.
Standby is becoming a more popular option as many witnesses work and cannot take the time off work to sit and wait all day at court.
For more information please see the websites in related information.