Whether the plan gets you there. These project forward through contributions, employer matching and growth, and show the answer in today money rather than a headline figure that inflation has quietly hollowed out.
The single most important thing a retirement projection can tell you is the inflation-adjusted answer. A projection showing a large future balance means very little on its own, because prices will have moved too. What matters is what that balance buys in today terms.
The savings account calculator models how a bank account genuinely pays: daily accrual, bonus rate periods, and the conditions attached to them. That is deliberately different from an investment projection, because a savings rate is a known quantity while an investment return is an assumption.