Express is a microframework that according to 2018 Node.js User Survey Report is used by 4 in 5 back-end and full-stack node.js developers. Thanks to its simplicity, the always-growing range of available middleware, and active community the express userbase is still growing.
Fixtures are a thin abstraction layer over sample data in your application which allows for better organizing, often complex, data structures representing different entities. If this sounds like a vague description or does not ring the bell maybe an example will speak to you better.
There are a few major reasons due to which you may find yourself creating a library. One, obviously, is that you have a solution which you would like to share with the Open Source community. The other one is that you need to reuse code across different projects or in the same project but on different platforms.