Robert Chambers wrote:
Sure it is relevant, until one IFR flight has cleared the part of the
airspace relevant to his departure the second IFR flight cannot be
released for takeoff.

At a towered airport you are talking runway separation. After the first
plane is 3000 feet down the runway the next one can be launched.



For the same reason a plane holding for IFR release cannot take off
until the IFR arrival he's waiting for has either cancelled IFR or has
landed.

No. The tower can provide visual separation if it doesn't have radar.