- "Every great advance in science has issued from a new audacity of imagination."
– John Dewey
The application of Scientific Theory - the principle that everything around us will work better if thought of logically - elevates science in your civilization to the next level through the Public School. It also increases Production from Lumber Mills!
Finally, it always allows the development of Steam Power and crucial strategic resource techs. Biology is required for Oil (and hence many Modern and Atomic era military vehicle units, including Tank types and planes). In the expansions it also leads to the cornerstone of modern industry Electricity, including to revealing the Aluminum strategic resource (although it can't be used for units until nearing the Information Era). Its role involving Coal and the Factory was replaced by Industrialization.
Scientific theory is a way to view the world, in which the viewer uses the "scientific method" to learn about the universe. Through careful observation and experiments a scientist creates a theory to explain some phenomenon. If other scientists can through experimentation confirm the scientist's theory, it is then accepted as "empirical" (experimental) law (at least until some new observation or experimentation successfully challenges it). If a scientist's experiments cannot be duplicated by others, then his or her theories must be regarded with deep skepticism.
Although prevalent in most advanced countries in the world today, scientific theory is not the only way that people look at the world. Some people look to divine revelation - as written down in a holy book, say - to explain the universe. If observation or experimentation conflicts with the revelation, then the observation or experimentation must have been flawed or corrupted. These two different methods of seeing the world have been in tension for centuries, and will probably continue to be so for years to come.