Your 're in a profession where even English doesn't mean what it says so don't ask me to improve mine.
If you are wondering how dry runs may help terrorists, then you haven't used your mind. Perhaps they were gauging response times, or maybe synchronizing their own timing. Who knows. Practice makes perfect.
No you haven't used your mind except, perhaps, in watching too much TV. Response time can be gathered by reading the newspapers, cops publish them for every city every year. IF you wanted to be exact do you really think it wise to call attention to a spot you are going to attack? If you move over a mile then your info is useless. IF you insist on doing a dry run than anyone with an IQ above 80 would use something that when the exercise is over will be thought not to have been real. A box with old books, a clock and a roll of wire. A briefcase with a travel clock, a flashlight, some old papers and $10, NOT a fake bomb.
We are in real danger, yes, but some of it is supposedly bright people jumping to stupid conclusions, time and time again.
Most of the plans made by terrorists depend highly upon timing, the synchronization of movement, communication between conspirators, etc. Don't center in on a particular locale only. Does our military run drills only in an area in which they know a battle will take place? Of course not. Like I said, practice makes perfect.