What you are likely to already know is that public transport is a cheap way of getting around, and even more practical for a built up area. However, it also has benefits that aren't mentioned directly.
- fewer fatal accidents from loss of control, drink driving, other road users
- trains don't need much space for high speed travel.
- don't need to waste time just to find parking.
- Depending on security systems in place and design, reduced likelihood of being robbed or hijacked while traveling. Especially when there are a lot of people around.
- Since you aren't using your ride, which itself can cost quite a lot, the loss is minimal.
There are negative things too, but when you prioritize the environment, they don't seem that important.