When ATC provide radar vectors for an instrument approach, they sometimes instruct pilots to intercept the localizer or final approach course before giving an approach clearance, and other times give an "early" approach clearance with the final vector for the interception while the aircraft is still heading to intercept the localizer or final approach course. What are ATC's considerstions when they "delay" the approach clearance until the interception of the localizer or final approach course?
The main reasons I will instruct a pilot to intercept and follow the localiser instead of clearing him for the ILS are separation and minimum safe altitudes.
In a busy approach environment, it is often beneficial to let two aircraft intercept the approach with 1000 ft vertical separation. Maybe the aircraft are slightly too close horizontally but the first one is faster than the second one. I can then have then both follow the approach course, and descent number two on the approach once horizontal separation exists.
As for minimum safe altitudes, if you are approaching an airport on a heading straight in toward the runway, maybe there is room for you to intercept the approach course a large distance from touchdown. However, at this distance it might not be possible to descend to the approach altitudes, due to terrain or airspace restrictions. In this case I will also clear you to follow the localiser and then give the approach clearance later.
There could be several reasons for this (more than I list below):
The controller wants the airplane to remain at its current altitude [perhaps for vertical separation reasons] and not begin its descent at that moment. (once the approach clearance is given, and the aircraft is established on the approach procedure, it can descend without further instruction as published on the approach chart)
The controller wants the aircraft to join the localizer and track inbound to the runway well before an approach clearance would normally be given. (even 20 or 30 miles from the runway)
The intercept angle the controller assigns to join the final approach course exceeds the permissible angle ( for airplanes, 20 or 30 degrees depending on certain factors). Sometimes, in order to achieve a bit more spacing between aircraft the controller may need a more "square" turn to final than would occur with a shallow intercept angle. Then once established on the approach course, the clearance can be given.
As I said, there are undoubtedly more reasons, but the three above represent some examples.