Not at a location most convenient for the train operator.
1. The card stores not only your ticket/cash data, but also your trips
2. You live far from the metro, so you ride by bus to the metro station. The metro station knows that you've just been on the bus (the bus activated 90 minutes window) and either don't account you or gives you a discount
3. You can also top-up the card using your smartphone with NFC. The smartphone writes data to the card (and can read your balance and a trip of course). But also, if your smartphone don't have NFC or you don't want to install the app, you can top-up using the web and write it to the card anywhere (inside the bus / on a metro station) with the regular validator (it not only reads, but writes the data received from the net as well)