Here's the formula for free cash flows I'll be referring to:
FCF = EBIT*(1-Tax Rate) + Depreciation and Amortization – Capital Expenditures – Increases in Net Working Capital (NWC)
If you have an increase in net working capital, you have more current assets than liabilities than you did in the previous period. So if you now have an increase in net working capital of, say, 10, why would you subtract this to get your free cash flow? Since current assets include cash, wouldn't you be subtracting an increase in cash (in some cases) from your free cash flow?