From the Readme:

Why does it exist? 🔓

Calibre, while a fantastic tool for its age, has several problems when containerised, including its reliance on a KasmVNC server instance for the UI, which is near impossible to use on mobile and is relatively resource-heavy if you’re running a small, lower power server like I am.

For many, Calibre-Web has really swooped in to save the day, offering an alternative to a containerised Calibre instance that’s resource-light and with a much more modern UI to boot.

However, when compared to full-fat Calibre, it unfortunately lacks a few core features leading many to run both services in parallel, each serving to fill in where the other lacks, resulting in an often clunky, imperfect solution.