An urban growth boundary has two main purposes: to rein in urban sprawl by promoting more compact development and to protect wild and agricultural and other resource land from low-density helter-skelter development.
Probably the best-known UGB is in Portland, Ore. As Dalhousie planning student Patrick Moan (once a Portland resident) notes, in the early 1970s sprawl had become so bad the governor said that "he hoped visitors would not move to Oregon after seeing what the state had to offer."
State law now requires locally prepared comprehensive plans to include a UGB. The UGB draws a line around the municipality and establishes where development can and cannot occur. A similar scheme in Maryland denies state funding of any kind (schools, transportation infrastructure, etc.) in areas not earmarked for development.
Greenbelts, which attacked the same problem less comprehensively, worked more by setting land aside than by putting restrictions on development within and beyond the belt.
But greenbelts tend to be legislatively weaker than UGBs, because the development restrictions are usually not crafted in stringent detail. The more simplistic idea of a "belt" has generally been replaced with the notion of corridors of open space and boundaries to development.