One easy way to make money from cooking apples is to pick them up at the Cooks' Guild (with 32 Cooking) and then sell them on the Grand Exchange. To increase the amount of money made per hour, you may pick up the chocolate bar (223 coins each) or grapes (1,826 coins each) also found in the Cooks' Guild.
The apple's examine of "Keeps the doctor away" is the latter half of the proverb, "An apple a day keeps the doctor away." A barrel of apples (non-carryable) completes the proverb with its examine of "An apple a day..."
When the cursor is hovered over any edible food, the icon for eating is an image of a bitten cooking apple. However, cooking apples cannot actually be eaten.