What reasons might make day-old pastry the preferred choice in these recipes?
It may be a matter of texture, rather than taste.

The recipes you link involve putting a wet paste on top of a previously baked croissant.

Most baked goods change in texture after sitting out for a few days, mostly because they get drier.  If you want to turn bread into crumbs or croutons, for example, you slice it and let it dry out before processing it.

By letting the croissants sit for a day or two, their surface will become harder, and they won't get soggy when the almond paste is applied.