GitHub-hosted runners have a limit on the number of concurrent workflows (typically 20 for free-tier accounts and 5 for enterprise). If your organization needs to run more workflows simultaneously, you would need to use self-hosted runners to increase the available concurrency.
Self-hosted runners allow you to configure specialized hardware or software setups that are necessary for certain workflows. GitHub-hosted runners may not have access to custom hardware configurations like GPUs or other specialized resources, so self-hosted runners are required in such cases.