technology

The 2 have overlapping but distinct purposes.

Obsidian manages directories of mostly plain text files. It has a huge number of plugins that allow manipulation of the text files. The emphasis is strongly on structure between and within documents. There is very little here to control how a file would look when printed. I am not even sure if printing is possible without installing something. Obsidian is about collections of files.

LibreOffice is an office suite. One of its components is Writer which is word processor and probably what is being discussed here. Most people use it more as a document layout software. Though it is able to work with document structure like creating a table of contents from headings, or manipulate data like incorporating excel spreadsheet in a mailmerge. The end goal is to create a document that is printable and to control that. You mostly work with files 1 at a time. The file format is much more complex to allow for all the details of page layout.

If you are not creating documents to be printed for the most part, stick with obsidian. It is more flexible.

Also worth saying that libreoffice is free libre and open source software. Obsidian is proprietary although there is a large open source community around it. Zettlr and Joplin are floss alternatives. I prefer obsidian personally.