The width of the spread reflects the fixed costs of the business, the risk of holding inventory, and the risk of trading with better-informed counterparties (adverse selection).
Consider a hypothetical example in which the bid is 9,990 yen and the ask is 10,010 yen. The spread is 20 yen. If a market maker buys at 9,990 yen and sells the same quantity at 10,010 yen, it keeps 20 yen per unit. If the price falls after it buys and before it sells, however, it loses, and if the price rises, it gains.