What are ATC's considerations when they instruct to intercept final approach course instead of simply clearing for approach?
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.