You may have heard the term "player trading." You sign a player for a million, he does well, appreciates in value, and you let him sign with another club for 10 million. You've made money and, for many clubs, this is an increasingly important revenue stream alongside gate receipts, media rights, sponsorship and commercial income.
In fact, as the financial blogger @SwissRamble writes, in the past decade (pre-COVID), the top two tiers of English football have lost a whopping £3.1 billion ($4.1 billion), pre-tax, which would have become a massive £8.2 billion ($10.85 billion) pre-tax if not for the massive (£5.