Few reasons - 1st, incase of delayed engine fire/jet blast & engines are still turning albeit spooling down, or 2nd, incase anything falls of the a/c & lands on you.
Regarding the 2nd point - There is a treaty under ICAO Aviation Law, called the Rome Convention (anyone care to correct me, been a long time since the ATPL exams), that has implications in case of such occurances.
Thats the 2 main reasons, but I'm sure there are others that airlines & airports will enforce & stipulte.
HPC