Useful possibilities appear at inconvenient times: call the accountant, learn a language, fix the chair, explore a new project. If every possibility enters today's plan, the plan stops expressing a choice.
Steady keeps unscheduled candidates separate. Later or Someday can hold work that may deserve attention without consuming today's capacity. The item can return when you plan a future day, where it competes honestly with other choices.
This is not a hidden backlog that must eventually reach zero. Some possibilities will become commitments. Others will remain possibilities or be released.
Today's plan should show what you intend to protect today, not everything you are capable of remembering.