Working capital is the cash tied up in the day-to-day mechanics of running a business — inventory sitting on shelves, money owed by customers (receivables), offset by money owed to suppliers (payables). Growth usually eats cash here before it shows up as profit anywhere else.
Why an increase in working capital is a cost. A growing retailer needs more inventory to stock more stores; a growing software company selling on invoice needs to fund a bigger pile of unpaid customer receivables while it waits to get paid. That cash has to come from somewhere — it's a genuine use of cash, even though it never appears as an "expense" on the income statement. Owner earnings subtracts it for exactly this reason.
Why it's not a constant drag forever. A mature, slow-growing (or shrinking) business often needs little or no additional working capital year to year, or can even release cash by shrinking it — which is one reason mature "cash cow" businesses can convert a higher share of accounting profit into real distributable cash than fast-growing ones.
The sign convention. Intrinsic treats an increase in working capital (a use of cash) as a positive number that gets subtracted — if a company actually released cash from working capital in a given year, that shows up as a negative value, adding to owner earnings instead.