They look like they're there in the case that somebody falls on the tracks, they can hide behind one of these if a train comes. Staying behind the fence will insure you're not hit by the oncoming train.
The third rail is covered. The yellow boards indicate where you can cross it. These yellow fences offer protection for those standing at track level when a train passes. There isn't anything to hold onto there.
Yeah, these are safe areas for those working on the tracks for when a train comes through. Although it might not be a bad idea, they're really, really ugly and hurt the already lacking asethetic of TTC subway stations.