Early tree guards were installed to protect tree trunks from errant carriages and horses. Horses would eat the bark of street trees. With curbs and motorized vehicles, the utility of guards have declined, though they continue to be installed to deter vandals and to decorate sidewalks. The traditional tree guard requires maintenance; as a tree trunk expands, it can can grow into the metal bars of the guard. On more consideration, guards can protect trees from cyclists who will lock their bikes to tree trunks in the absence of proper bike racks.
Now guards are offered in shorter and alternative styles. For example, Arlington County, Virginia has installed low-slung, decorative guards in metal (the traditional material), and in concrete and brick. The latter are not tree (trunk) guards per se, but they do hinder access to tree wells (pits, parkways, bed space).
None of these guards are multi-use. But New York's tree guard bike racks are. A 2001 design competition sponsored by Trees New York and Cooper Union yielded two fabulous multi-use designs. Last fall's CityRacks Design Competition yielded 10 finalists, one winning bike rack design, and no combination tree guard bike racks.
Locally, I have come across three multi-use tree guards. Two are designer guards; one of which doubles as seating, and the other as a bicycle rack. The third is a do-it-yourself guard. I think its additional purpose is to alert cars to the presence of the curb.