The notional is a reference for calculating payments, not an amount at risk. A $100 million interest rate swap exchanges interest calculations on $100 million; only the net difference changes hands, typically a few hundred thousand dollars a period.
Swaps let a user change an exposure without touching the underlying asset. A borrower with floating-rate debt can fix its cost with a pay-fixed swap while leaving the loan in place; an investor can gain index exposure without buying shares. That separation of economic exposure from legal ownership is the family's defining usefulness.
Because they are contracts rather than instruments, swaps carry counterparty-risk and are governed by an isda-master-agreement. Standardised types are now largely routed through central-counterparty clearing.
Related: interest-rate-swap, total-return-swap, counterparty-risk, isda-master-agreement, central-counterparty, notional-value