A free bet is promotional credit: place it, and if it wins you keep the winnings but not the stake itself ('stake not returned', SNR). A £10 free bet landing at 4.00 pays £30, not £40. Because the stake is never yours, the token's real value is roughly its face value times (1 − 1/odds) — which is why free bets extract more value on longer prices.
Free bets are the currency of welcome offers and promotions. Used carelessly on short favourites they return a fraction of face value; used deliberately at odds of 3.00–5.00 they convert to 65–80% of face value in expectation. Always read the attached terms: expiry windows, minimum odds and market restrictions decide what the token is actually worth.
£20 free bet at odds 5.00: wins £80 profit 20% of the time → EV = £16, i.e. 80% of face value. The same token at 1.40 returns just £8 × ... ≈ EV £5.70 — under 30% of face value.