I started poking around and found this thread on the Gitane USA Forum. It makes as much sense (or more, actually) than other explanations I've seen.

It seems some French manufacturers used less expensive straight gauge tubing for the steerer tube (rather than butted tubing) and then brazed in a split sleeve at the crown end to strengthen the steerer to crown joint. The structural problem with this approach is that in a crash with lots of stress on that joint, the split sleeve brazing could fail leaving just the thin straight gauge steerer tube wall, which would be more likely to bend or break off at the crown. Bad news.

So, to prevent the split sleeve braze from failing, the wooden dowel was tightly fitted to the bottom two or three inches of the steerer tube.