The rule is being implemented for two reasons, one overt and the other more subtle.
The overt - at busy stations such as Union, it can take a full minute (or more) for the CSA to close the doors on a train, especially if it is crowded and both sides are open. Closing the doors "early" - in the sense that they start the door closing procedure one minute prior to scheduled departure - is a means to help the trains get out of the station on time.
The subtle - this is also a means to get passengers aware that they need to get to the stations earlier, so that they are not rushing the doors as they close. This is why GO's messaging on this has been fairly quiet and with few clarifications. If they can get people thinking that they need to be at their home station - regardless of whether it is actually affected by the rule - earlier in order to make sure that they get on before the doors close, then all of the "bad press" has done its job.
It should also be noted that most systems around the planet have some similar rule already in place. Intercity systems will have such a rule at all stations, or at least the major ones, while commuter systems will do it only at the point of origin for a specific trip. The amount of time varies somewhat based on their operating practices and cultural details.
Dan