a public vehicle running regularly along certain streets, usually on rails, as a trolley car or trolley bus.

A streetcar is one of the oldest forms of mass rail transit. In the early 19th century streetcars were pulled by horses and by the end of the 19th century electricity had taken over as the power that ran the cars.

Streetcars (or trams) are a heavier use replacement for busses along a corridor that supports higher density. Since streetcars operate in mixed traffic along with other automobiles (the same as busses) it is subject to much of the same traffic congestion that automobiles are but areas close to a streetcar line typically exhibit a much higher density than those that aren't. Because of this propulsion of density surrounding a streetcar line, the additional taxes brought on by the higher density helps to justify the costs associated with such a mass transit option.

Since streetcars operate in mixed traffic what is the advantage of having this type of mass transit option?
Higher density can be supported by a streetcar than can be done with a bus line along the same route. Perhaps it is an idea only in the heads of developers, but they are much more likely to build higher density along a streetcar line than a bus line of the same route resulting in a higher tax return for the city helping to offset the costs associated with the line.