Any workspace can be linked to a parent, turning it into a "phase" of a bigger initiative — a numbered feature underneath a broader roadmap, for instance. Phases show up nested beneath their parent in your workspace list (see Keeping a growing workspace list tidy), so a big effort stays visually grouped without losing each phase's own independent history.
A phase workspace can also share its parent's checkout instead of getting its own — useful when a smaller piece of work genuinely belongs on the same branch as the thing it's part of, rather than isolated on its own.
None of this is required for smaller work — most people never need more than a single flat workspace. It's there for the moment a project outgrows that.