Downsview Park GO is also great because the Barrie line previously had no stop in Toronto with connecting transit prior to Union. The York U GO stop is in the middle of nowhere, and a bad neighbourhood, a fairly long walk just to get to a bus route with a long trip to the subway. If you lived on the Barrie Line and wanted to get to the north end of the city, it made much more sense to drive than take the train all the way down to Union and take an hour-long trip back north. But now, Downsview Park opens, and there's a direct, roughly one minute walk inside the station to the subway platform--anyone who wants to go to the VMC area, Yorkdale, anywhere along St Clair via the 512, and I'd argue any students at U of T St George or anyone needing to go to Yonge/Bloor or anywhere on Line 2 will all benefit from exiting at Downsview Park and boarding a non-crowded subway there, rather than going all the way to Union and backtracking up on a busier subway.
Edit: played with some transit directions on Google Maps. York U-Union is 22 minutes by GO, so Downsview Park will probably be 20 minutes. As an example, the subway trip to St George is 19 minutes from Downsview, so probably no more than 22 minutes from Downsview Park, and we can round that up to 25 minutes factoring in wait time for a train. Arguably it would take you at least 5 minutes to get off GO at Union and walk over to the TTC station and get down to platform level, so that makes it 25 minutes from Downsview Park GO-Union TTC. The subway trip from Union to St George is listed as 8 minutes. So, anyone going to St George would likely save nearly 10 minutes each way by transferring at Downsview Park rather than Union--that really adds up in the course of a year.