[riskable]

Because of shades surrounding the traffic lights (to prevent glare from the sun) it is often impossible for pedestrians to see if a turn signal is lit from their perpendicular position.

So in the event of a red light for through traffic but a green light for turning traffic you don't want pedestrians crossing the road. That's why it is important to have separate lights indicating when a pedestrian may cross.

[ptaipale]

Right - and even without any shades, there are places where I can't see at all the light that permits vehicles to cross my path.

For instance, if I'm crossing the road here https://goo.gl/maps/favpGxyKTLk (this is on my daily cycle commute)

there is no way for me to know when there is a green arrow light for vehicles that come from opposite direction and turn left (and therefore come my way).

The buttons requesting green man for pedestrians and cyclists is, btw, known here as "humiliation button" (because it forces cyclists to stop; there is automatic detection for motor vehicles on the driveway, but for pedestrians and cyclists they haven't found a reliable way to do it).