It is used both during build and run time.
During build time that variable,the STRAPI_LICENSE var, and NODE_ENV are the only three used and compiled/built into the React admin panel (the license one only applies to EE users). The url key/env var is also used during runtime within the Strapi Koa.js backend to modify variables of various plugins such as
- Users-Permissions plugin for 3rd party auth callbacks and email templates
- Upload plugin (local provider) for file uploads
- strapi-admin package for admin related emails and links
- various other parts of the Strapi code-base