My use case looks roughly like this: for a given project (as in hobby/DIY/learning, not professional work), I typically have general planning/reference notes in a markdown file synced across my devices via Nextcloud. Separately, for some individual tasks I might have comments about the initial problem, stuff I researched along the way, and the solution I ended up with. Or just thinking out loud, like you mentioned. Sometimes I'll take the effort to edit that info into my main project doc, but for the way I think, it's sometimes more convenient for me to have that kind of info associated with a specific task. When referring to it later, though, it's really handy to be able to use ripgrep (or other search tools) to search everything at once.

To clarify, though, Vikunja doesn't have a built-in feature that exports all task info including comments, just a REST API. It did take a little work to pull all that info together using multiple endpoints (in this case: projects, tasks, views, comments, labels). Here's a small tool I made for that, although it's fairly specific to my own workflow: https://github.com/JWCook/scripts/tree/main/vikunja-export