Significantly less disk usage is the main one for me. If you have a workflow where each task has a worktree, and you have 10-15 tasks in progress, and the repo is a decades old monorepo, then storing only one copy of the history is a big savings.

A task could be any small thing that requires its own checkout for some possibly stupid reason, like running the regression tester or temporarily running an old version of a binary out of your dev checkout over NFS if there was a bad deploy.