An example for b)
VOR Approach Rwy 28. (no DME). VOR located at 5 NM from the threshold 28.
Arrival making straight in. Departure to go RWY 28 on a track 280.
First what separation could you use without this one? Probably 10 mins longitudinal climbing same track is all you can find. Bit restrictive.
With the VOR as the designated point this allows you to get the departure airborne prior to the arrival checking the VOR. 3 minutes instead of 10....
This separation has probably been in the 4444 since the first day. (When DC6's ruled the world and NDBs were the main aid). Someone would have decided that this was safe.
You would have to watch the missed approach if the departure was slow.
In the new RNP/RNAV world I can imagine (in a procedural environment) that this will still apply with a point designated on the approach after which a departure cannot be released. Of course the computers will be managing that and it will vary for each combination of flights!
