I just realized while walking around today that only select intersections have these buttons. Another thing I realized was that not every corner in that intersection got one, only three out of four did. So how do they decide which streets get the buttons? And how do they decide which corners of that intersection get buttons? Thanks for the replies. 
One of the main criteria is whether the light will remain green long enough for even a slow pedestrian to make it across the intersection in time. If there are no automobiles entering from the side street, active control systems will usually cycle the light back against cross traffic rather quickly. Often, this does not permit enough time for a pedestrian to cross. The “Push-for-Green” button will impose an extended time period sufficient for a pedestrian to make it across.
At really large metropolitan cross streets, there is often one of these same buttons located on the midpoint island for those who only walked halfway across on the light. Also, modern processor controlled traffic signals are often set to remain green for the main boulevard and change only when a car arrives from the feeder avenue. The “Push-for-Green” button permits a pedestrian to cycle the light instead.
So how do they decide which corners of an intersection will get them? As said before, some intersections where I live only have three of the four corners with the buttons.
I think Zenster already nailed it. If the intersection has a main road and a ‘feeder’ road, generally there are buttons set up for crossing the main road, since it would generally be more difficult to cross the main road without the aid of the button.
If you’re talking about one of 4 corners (say the north, south, and west have buttons but the east corner does not), my guess is that it’s either due to the Powers That Be deciding that pedestrian traffic on the one corner is not high enough to justify a button or a mistake/omission during the construction of the lightpoles. Or possibly the lightpole without buttons was a replacement for one that was destroyed in a car accident, and unfortunately the replacement doesn’t have the buttons included.
It’s generally a matter of pedestrian/vehicle volumes. If you have a number of pedestrians crossing one side of an intersection, you may install the buttons only along that side because the volume warrants it. Similarily, if you want to encourage pedestrians to cross on a specific side (perhaps to discourage an unsafe road crossing at another point) you may place a button only on one side of the intersection, attracting pedestrian traffic to that location.
Give your local Public Works department a call. They may be able to provide more specific reasons for a particular location.
–Patch