Originally Posted by
IanW
It's a piece put there to close the mouth up a bit.
Wooden plane soles both wear & can get damaged with use, & need re-flattening from time to time. Because the front of the throat slopes forward, this will gradually increase the gap between the cutting edge & the front of the mouth. When that gap becomes too much to be acceptable, the solution is to insert a piece of wood as demonstrated, to close it up a bit. It just indicates your plane has had a long & honorable life.
A 'fine' mouth is more important on smoothing planes where you really need to control tear-out, and in fact, you need plenty of room to allow large shavings through on scrub planes & roughing jacks. You can also control tear-out by setting the the 'chip breaker' very close to the cutting edge, and many people think that works better than a fine mouth.
Cheers,