Because parking is presumably cheaper on the periphery and the subway may actually be faster than driving all the way in. When I worked in Boston, that's how I sometimes did it. (Other times I took the commuter rail but that was more or less useless if I was doing anything in the evening.)
To use an example from a German city: Because going from the park+ride spaces at the edge of the city to the center takes > 45min by car, but 4min with the train. And parking at the edge of the city is free, but downtown you pay 8,50€/h.
It actually makes a ton of sense. Imagine a scenario where parking downtown were actually impossible.
You would then want to drive as far as you can, park, and take the alternative in.
It's made for people that don't live walking distance from a train/bus line, but also don't want to deal with the traffic/cost/overall hassle of going by car end-to-end.
There are 80 Park‑n‑Rides throughout the my metro area.
