One of the nice things about rowing indoors is that the repetitive motion in a safe environment allows you to think without having to pay attention to your environment for safety. While you can think about your technique or focus on your pace, you can also let your mind wander.
Unlike running, there are no curbs or loose rocks to be wary about. Unlike cycling, there are no cars or other moving bodies that can collide. Unlike outdoor rowing, there are no waves to contend with or fishermen or other rowers to watch out for. The indoor rowing motion repeats and you sit, tied in, in your own environment.
Some rowers enjoy focusing on their row, their pace, their technique, the feel of the parts of their body working in coordination. Certainly, if you are training to improve your pace, then thinking about what you are doing - or, as has become current - being mindful of your rowing can help you achieve more.
But a 45 minute row can also be a time to work through a problem mentally. Or to get over an annoying or troubling incident in your day. Rowing can be almost meditative in the way it allows you to let thoughts and emotions come and go. This aspect of rowing may explain another aspect of the feeling of being refreshed you have when you are done rowing. You breathe more deeply and get the blood flowing, waking up and restoring the body; and you let the mind restore itself, as well.