These options allow you to run playbooks from a certain part not from the start, they are very useful for testing new plays or debugging!
With "--start-at-task" you can start a playbook at a particular task, as following it will start from that task called "xTASK" regardless its place in playbook.
ansible-playbook playbook.yml --start-at-task="xTASK"Other option is "--step", which will ask you before each task, and you can choose if you want to run this task or not.
ansible-playbook playbook.yml --step Perform task: xTASK? (y/n/c):You can use both of two options in the same time, so it will start from certain task and ask you if you want to run it or not.
One more thing, however "start-at-task" is really handy, unfortunately it become less important in complex playbooks! Because if this task depends on another previous tasks it will fail or skipped! But as usual, it's better to know than not!