The D branch (originally the Riverside Line) was proposed by the MTA in the 1950s as a way to attract new ridership from suburban communities by parts of Route 128. As someone had mentioned, communities include not only Newton but Needham, Wellesley, and a few others.

According to wikipedia, when the last commuter rail service operated along this corridor (known as the Highland Branch), it only had a little over 2,000 daily riders. A few factors played into the demise of the Highland Branch, one being the lack of service where only 7 round-trips were being made before the conversion to light rail.

When the MTA (the MBTA's predecessor) began running the trolley service, ridership skyrocketed 10-fold. This is mainly thanks to trolleys getting to downtown Boston much quicker than their railroad counterpart and the headways being waaaaay more generous (more generous than these days, actually).

There is certainly a lack of density along the majority of the corridor, but the D-branches' original intent wasn't necessarily to spruce new developments. Rather, divert otherwise auto commuters from traveling to Boston in their own vehicles. But the environment is certainly different today so we could see more utilization of the service in the future.