When rowing there isn’t a marked difference though the grommet does seem to hold the oar to the pin more firmly. Comfortable rowing depends on the grommet or estrop having the correct tension—too loose and the oar’s all over the place. Tension varies as the rope stretches and the amount of stretch depends on the type of line and how wet the rope is. The estrop is adjustable and, having three turns of rope, stretches less than the grommet but a well-made grommet shouldn’t stretch beyond what is normal for the type of rope.
The most obvious advantage of the estrop over the grommet is that, being attached to the oar, it’s more difficult to lose. The other advantage is that when the oars are taken off the pins and stowed in the boat for sailing in rougher weather, the estrop, with its long trailing end is good for attaching the oars to the thwart.