They are the most precise end of a gradient of usefulness for subdivision of the Earth's stratigraphy. This sounds like cyclic to me reasoning to me. No, it is not cyclic reasoning. It isn't even close. This is a broad distinction which only separates Paleozoic from Mesozoic rocks, but it is still consistent world-wide, and individual species of trilobites or dinosaurs within that interval might be used to establish finer subdivisions.
Planktonic organisms turn out to be particularly useful.
There is also ample independent evidence for the correlation of time intervals from one area to the other on the basis of fossils. For example, volcanic ash beds with precise, distinct chemical compositions can often be traced laterally over large distances. In fact, there is little in biostratigraphy which depends upon evolutionary theory. John Smith was subdividing the rocks of England using fossils decades before Darwin proposed evolutionary theory. They are only used to establish the subdivision of the Earth's stratigraphy.