Branch Avenue is the last station on its line. The whole DC Metro system treats the last stop as a major park and ride station, on all the lines, with ample parking and a big bus terminal. Some of them also have connections for the VRE or MARC systems, or even Amtrak stations for intercity rail.

It's for when people from the suburbs want to go into the city but don't want to look for or pay for expensive parking.

So the debate at that point becomes: do we want the system to be available to the residents of non-walkable neighborhoods so that they can still come into the city for work or entertainment without adding to car congestion and parking issues? For a city like DC that often has high attendance events, where hundreds of thousands of people show up for a day, I think that's probably worth it.

The last station on any given line doesn't need to be walkable, in those cases. Most lines in Chicago have parking at the last station on the line, as well (and many have connections to commuter rail with Metra, which definitely have parking at their stations). Even the end of line stations that don't have parking in that system are pretty centrally located, showing that tradeoff on whether you want to build the line all the way out to the suburbs or not.

New York has lots of walkable stations at the end of line, but it also sits in an urban core where the metropolitan area doesn't actually fall to low density, so that the system does have to hand off passengers to the commuter rail lines (Metro North, LIRR, NJ Transit) that are also in heavy use, but that tend to have much demand fluctuation by time of day or day of the week.

But when discussing individual stations, it's important to make sure you understand whether the concepts you're discussing are common or unique to that station, and why things were done that way, so that you can evaluate whether it was good or bad that things were done for that reason.