You often see claims that transit project X will reduce/alleviate/eliminate traffic congestion. E.g. This article discussing the issue
First this is usually wrong. Most transit users would not otherwise drive themselves. E.g. The 2004 Metrotransit strike could not be detected in the traffic counts. Most displaced users carpooled, walked, or biked. That is not to say there was no hardship, there was, but it was felt by transit users not drivers.
Second congestion reduction is not a good reason to support transit, anymore than claiming a new road will reduce crowding on the bus.
The purpose of transit is also not economic development, though that might occur as some people will pay a premium to live in places or work in places with good transit service. This indicates a willingness to pay above the fare being charged as well as an option value.
The purpose of transit is moving people from A to B who want (and/or don’t have a good alternative) to use transit. Their willingness to pay for that trip is the primary (and perhaps dominant) benefit transit provides.
All else is noise.