ardens and plazas share a simple purpose: They make their surrounding neighborhood nicer. Of all the many things you could live next too, wouldn't gardens be a top choice? Gardens don't need road access, a water supply, laborers or maintenance. They cannot catch fire or collapse. You can build gardens by placing them one at a time, or by clicking and dragging a larger area.

Plazas work the same way, except that they can only be built over paved roads. Even paved roads are utilitarian, meant purely for moving traffic as efficiently as possible. When you replace a road with a plaza, you cover the plain flat paving stones with mosaic tiles, adding beauty and instilling civic pride in what was, before, nothing but a busy thoroughfare. Plazas will not actually reduce traffic or make the former road any less functional. They merely make a statement about the value of the properties by which they pass.

Both gardens and plazas are extremely valuable tools for governors to make their cities nicer. Both have valuable desirability effects, and are all the more useful since they can be placed as a single small plot, or used to fill a larger area.