A certificate of deposit pays a rate fixed for a stated term, and the depositor gives up the ability to withdraw freely. The early withdrawal penalty is what enforces that exchange.
The bank is buying predictable funding
Banks fund loans with deposits, and loans have long maturities. Funding them with balances that can leave at any moment creates a mismatch between how long assets and liabilities last.
A term deposit reduces that mismatch by committing the money for a known period. In return, the bank pays more than it would on an account the customer can drain instantly.
The premium over a savings account is therefore compensation for the depositor's lost flexibility, not a reward for choosing a better product.
Penalties are expressed in interest, not principal
Typical penalties are stated as a number of months of interest, scaled to the term. Short certificates carry smaller penalties than multi-year ones.
Because the penalty is measured in interest, withdrawing very early in a term can reach beyond what has been earned. Institutions may then deduct from principal, and disclosures state whether they will.
Penalty terms are set by each institution rather than a national standard, which is why the same nominal term can carry meaningfully different exit costs.
Rate movement decides whether the penalty binds
If prevailing rates fall after purchase, the certificate's fixed rate looks favorable and there is no reason to break it. The penalty never comes into play.
If rates rise, the depositor is holding a below-market rate and may want to move. The penalty is precisely what makes that move costly, and its size determines whether the switch is worthwhile.
That asymmetry is the essence of the instrument: the depositor absorbs the risk of rates rising in exchange for certainty if they fall.
Maturity and renewal have their own rules
At maturity a grace period opens, commonly a short window during which funds can be withdrawn or moved without penalty. Missing it usually triggers automatic renewal.
Renewal is generally at the institution's current rate for that term, which can differ substantially from the original. A certificate opened during a period of high rates may roll into a much lower one.
Variants alter the trade in specific ways
No-penalty certificates allow withdrawal after an initial period at a lower stated rate, which prices the flexibility explicitly. Bump-up versions permit a one-time rate increase if the institution raises its offering.
Laddering, in which deposits are split across staggered maturities, addresses the same problem structurally by making some portion available at regular intervals without any penalty.
Each approach is a different way of buying back the liquidity the basic certificate gives away.