737 arming A/T while stationary at aerobridge.
Until Boeing changed it's policy, the arming of the autothrottle was left until approaching the take off point. Presumably this was a safety feature in case of an inadvertant touching of the TOGA button (s) on the throttles while taxiing.
That being so, one wonders the rationale behind Boeing's amended requirement to switch on (arm) the autothrottle during before start checks. Surely it would be safer to err on the side of caution and not arm the autothrottle until it is safe to do so as per previous Boeing recommendation - particularly as an inadvertant pressing of either TOGA button after engine start could have disastrous consequences. I understand some Boeing models have a safety device that inhibits the autothrottles from operating unless certain parameters are met but certainly in the case of the 737 Classics there is no such technical protection.