in_place_editor automatically calls a controller named set_table_field. You can create one automatically with teh in_place_edit_for but only if ey want to display the actual text typed. That doesn't work for drop-down boxes or in cases where you want to update other elements based on the field changed.
For that you create the method yourself, and rely on the params[:id] and params[:value] to find the correct record, update the attribute, save and return the value to be displayed via ajax. If you have a lot of them in your site, you end up with a lot of code duplication. This helper fixes that.