Hi yangnigon,
I personally (as I'm a lazy guy) always run my BPMs with the default setting, which is under the local system account.
The usual reason for running BPM or at least one instance of BPM under a certain user is to enable BPM to execute scripts which require a certain user to be logged in to successfully execute the script.
The other reason is security, as mentioned in the manual, so that BPM doesn't run with more permissions as required.
Greetings
Siggi