Cash out lets you close a live bet before the event ends: the bookmaker offers a buy-back amount based on the current likelihood of your bet winning. Winning positions can be banked early; losing ones salvaged in part. It feels like control, which is exactly why it is marketed so heavily.
The mechanics are less friendly: the cash-out quote is the fair current value of your ticket minus a fresh margin — often larger than the margin on the original bet. Routinely cashing out therefore leaks EV twice. If you find yourself wanting to trade positions actively, an exchange with transparent back/lay prices does the same job at honest prices; if you hold value bets, the discipline is usually to let them settle.
Your £10 at 3.00 is 70% to win late in the game. Fair ticket value = 0.70 × £30 = £21. A typical cash-out quote of £18.50 quietly charges you £2.50 — a 12% haircut for the button.