Track loaders offer a hell of a lot more traction than wheeled versions and have low ground pressure thus raising the floatability. Although the versitility is not as great as wheeled versions, sometimes jobs require those certain capabilities. That's why also track models are a minority. There is also the fact of operator preferance.
I would say that the DPW keeps theirs at the stockpile yard because they don't use it as ofen on certain jobs. By doing this they can ensure there is a loader there most of the time for loading trucks. Another reason could be that it is much better to stockpile materials with a tracked loader vs. wheeled.
Hope this helps...