Here be your applications!
a controlled environment - any increase in the number of files (non php-temp files) in your jail is highly suspicious
use extra filesystems for data, use noexec if possible
check for any modified (mtime 0) files in the jail-os area.
ensures separation of i.e. mysql and apache
but don't use it with pgsql - shared memory is not jailed (unless you only use shm for pgsql)
latency increase minimal
don't run anything on the jail host, only allow vpn'ed ssh access.