Searching for patents by their classification number is sometimes more accurate than searching by keywords. You can also optimize many patent searches with a combination of keywords and classification numbers.
This article illustrates the role of the patent classification scheme in searching for patents.
Using Patent Classification To Discover Chemical Information in a Free Patent Database: Challenges and Opportunities, by Stefan Hartinger and Nigel Clarke, Journal of Chemical Education (2016) Volume 93, pages 534−541, DOI: 10.1021/acs.jchemed.5b00740
Training material for this classification system which is now used by both the United States Patent and Trademark Office and the European Patent Office. This classification has also been retroactively applied to over 90% of older patents.