In early game, you do spend some time using vanilla furnaces. May as well make use of that heat and create some materials. That's what the Seeback Gen is for. It's a generator that makes a small amount of RF from your vanilla furnace operations you would do normally (like making charcoal until you unlock coal).
Of course you can provide RF any way you like to the Resource Generator once you get access to other machines. For example, the Furnace Generator is a very early and very inexpensive recipe, making it easy to make a few for faster resource generation.
