What is a frying thermometer?

The reason it's necessary for serious baking is that often an oven thermostat doesn't represent the exact oven temperature, and often different spots in the oven can vary 5-10 degrees. This can impact (actually ruin, in some cases) the chance for success, depending on the delicate nature of the project. Baking is really just a big exercise in chemistry: measurement, chemical reactions, catalysts, etc... so the precise heat can be a make-or-break issue.