Protecting a recently planted tree can be the deciding factor in its success. In some places traffic can be so heavy that even fully grown trees need protection.
Trees in the city can be vandalised and damaged by bikes, cars, and garbage trucks, people celebrating, pets, etc. It might be a good idea to found a permanent protection for trees in areas of such heavy traffic.
A visible protection in the shape of a tree guard and a tree grille is part of urban landscape and thus serves an important design function.