While there is a general predisposition against pedestrian bridges, for the reasons noted above (killing off street level life and retail, often tied to car parking directly adjacent to off-street development), there are some cases where it works.
It helps with getting folks across dangerous highway-like roads, but getting people to go up and over is hard. If the grade crossing is not sufficiently scary, they skip the bridge.
A pedestrian bridge can connect important existing above-grade nodes that are separated, usually by roads or transit. A good example in my town, Boston, is the bridge between Copley Place mall and the Prudential Center mixed use development. These two developments are period pieces that you wouldn't build today (isolated, auto oriented), but given their nature the ped bridge works well.
Pedestrian bridges can be useful in campus settings, allowing connection between academic areas that promote collaboration that might not take place in entirely spearate buildings.