A certificate of deposit usually pays more than an ordinary savings account at the same institution. The reason is not generosity but the term commitment, and the early withdrawal penalty is the mechanism that makes that commitment real.
The bank is buying certainty about duration
A savings balance can leave at any moment, so a bank has to assume some portion of it might. That uncertainty limits how confidently the money can be matched against longer-dated lending.
A certificate removes the uncertainty by contract. The bank knows the balance will remain for a stated number of months, which lets it plan the asset side of its balance sheet against a known date.
That knowledge has value, and the premium over the savings rate is what the bank is willing to pay for it. The longer and firmer the commitment, the more the certainty is generally worth.
Without a penalty the commitment would be meaningless
If a depositor could withdraw freely, a certificate would simply be a savings account with a higher rate, and every depositor would take it. The bank would gain nothing it could rely on.
The penalty converts a promise into a cost. Breaking the term is permitted, but it surrenders a defined amount of interest, which discourages withdrawal for anything other than genuine need.
Penalties are typically expressed as a number of months of interest rather than as a percentage of principal, and the length of that forfeit usually scales with the term of the certificate.
Interest rate risk sits behind the whole arrangement
When a bank accepts a fixed-rate deposit for a fixed term, it takes on the risk that prevailing rates move against it before the term ends. That risk is priced into the offered rate.
If rates rise afterward, the depositor is locked into a rate that is no longer competitive while the bank benefits. If rates fall, the reverse is true and the depositor holds the better position.
The penalty prevents depositors from resolving that trade one-sidedly by walking away every time rates rise. Without it, the bank would carry the risk in both directions and would not offer the premium.
Ladders and step structures manage the lock
Because a single long certificate concentrates the commitment, some depositors split a balance across several maturities so that a portion becomes available at regular intervals.
As each rung matures it can be renewed at whatever rate then prevails, which spreads exposure to rate changes rather than betting the whole balance on one moment.
Some institutions also offer certificates with a one-time rate adjustment or a reduced penalty, which trade away part of the yield premium in exchange for more flexibility.
Automatic renewal is the term most often missed
Many certificates roll into a new term automatically at maturity unless the depositor acts within a short grace period stated in the account agreement.
A balance that renews starts a fresh commitment, complete with a fresh penalty, at whatever rate the institution is offering at that moment rather than the original one.
The grace period is therefore the point at which the whole arrangement is genuinely open again, and missing it restarts a lock the depositor may not have intended.