While travelling on the local Copenhagen train system, I was curious to find plastic bags at the back of each seat.
Why are these kept there / what purpose are they supposed to be used for?
Firstly, that's not a local train - it's the international train that links Denmark and Sweden.
To answer your question: I asked about this when I was travelling on the Øresundståg a couple of years ago as I wanted somewhere to put trash. The train manager told me I can use it for "whatever I want" and that the bags are there as a convenience for customers and to help reduce the amount of trash left openly on the train.