To row my single scull, I must get to the boathouse, carry my oars and shell down to the dock, fasten the oars in the oarlocks, shove off, row my workout, return to the dock, remove the oars, lift the boat out of the water, dry off its hull, and then stow the oars and shell. With the ergometer, I simply go to the club, jump on the erg, row, and get off. An hour on the machine consumes far less real time than an hour in the boat. Training in a single scull, I have only an approximate idea of how far I have rowed or how fast I am going. But the machine's electronic performance monitor spews out exact measurements.
Outdoor times are less meaningful.
Although rowers enjoy talking about their times over given distances and course records at certain regattas, everyone takes these numbers with a grain of salt, because wind, weather, current, and a host of other factors invariably influence the times recorded. Balance is irrelevant indoors, because the machine sits on solid ground. Hurtling my mass toward the stern will generate momentum that counteracts forward progress. The ergometer, too, has a sliding seat, but, there, the only thing I am moving is digits on a monitor, so I can roar up the slide with abandon. While I want to apply maximum force to the oar, my body weight also represents most of the load in the boat and can become ballast.