Why is the default branch selected as default value on Files and create merge request?

Question

In the GitLab in Files, create a branch or create a merge request I see the default branch was selected. What use of given this branch as default value.

Answer 1

While the default target branch is, by... default, the default branch of the repository, don't forget that, starting GitLab 13.11 (April 2021), you can set another branch as the default target for merge request.

That way, you can setup merge request which would target your own fork instead of upstream, if that is what you need.

In both cases, the use is to establish a default target branch for your MR.
When a merge request uses an issue closing pattern to close an issue, the work is merged into this branch.