Because when properly prepared, these make a flour as tasty as Chestnut flour. The trick is twofold: dry the acorns without overheating them, then use a slow cold water process to leach out the tannins. More on that in a later post. For now, gather them and spread them in thin layers in dappled sun at most, with good air circulation, for several weeks until the nuts are dry and hard.
We shell them when dried and discard any with problems. The good nuts get dried more in a warm (not hot) air fan. We gather in October and November with drying done by late December most years.
The abundance of bedrock acorn mortars in our area was my first unsubtle hint that these might be worth the effort. After several false starts, we seem to have this down to a stable process.
In addition to Tanoak (Lithocarpus densiflorus), the Canyon, Valley, and Blue Oaks have excellent acorns - they all get the same treatment.