It is useful in a pull request build job have the ref spec of the target branch of a pull request.
This is required to perform a git diff (or changelog) between the HEAD commit of the pull request and the target branch (master, main, dev, production, support/1.0.x ....).
Normally target branch is available when the merge strategy is set to MERGE, but it is not with HEAD.

I was asking this improvement for because the branch analysis performed with Sonarqube requires to have also the target branch of the to discover only the changes source code and perform analysis only on delta (speed up analysis) and show difference between the overall code against new code.