A steenblik coil is a spiral mirror that can focus the sun. A good mirror like gold (gold foil is cheap for optics because it can be beaten to a micron thick between calfskin and it reflects very, very, well) would heat an area a few feet across to the melting point of glass or metal very quickly for six to ten hours every day.
You can use it for:
Replacing wood for lime, mortor, or brick production. Cheaper houses.
Replacing wood for glass production. Glass is hard to make. Cheap glass windows is important for replacing vast amounts of wood.
Replacing wood for ceramics production. Cheap pots of all kinds.
Replacing wood for iron smithing. Not for smelting iron, but for heating it up and forging it.
Replacing wood for sulfur production. You can sweat the sulfur out of volcano rocks by heating them. That's how we got sulfur historically.
What else is it good for?
What would the Greeks or the Persians or the Carthaginians have done with cheap glass?