This action deletes cancelled workflow runs, which comes in handy when you use concurrency groups to limit a workflow to one run at a time, like this:

concurrency:
  group: foo

Typically, if you were to do this you would get a bunch of red ❌s on your branch for all runs cancelled by the concurrency group. In fact, there are two open issues related to this:

• GitHub Actions | Listing workflows cancelled by concurrency settings as failed could be misleading
• Cancel a workflow without considering it failed?

If you enable this action at the beginning of your workflow:

jobs:
  delete-cancelled-runs:
    runs-on: 'ubuntu-latest'
    steps:
    - uses: 'MercuryTechnologies/delete-cancelled-runs@latest'
      with:
        workflow-file: 'main.yml'

… then it will delete any recent cancelled builds so that they don't pollute your commit timeline with uninteresting failures.