A leveraged buy out like the one Bain Capital used here is often used to milk cashflow and value out of a business that is in a bad position. They essentially seized on the opportunity that Toys R Us' bad decisions left them in to buy them with debt and use them as an ATM. I suspect that Bain didn't care about letting them grow, and saw this as an opportunity to bleed out all value from them for their remaining years until they could file for bankruptcy and liquidate.
No, the debt is intended to give them leverage. Have $300 million but want to get the returns of having a $3 billion company? Use debt to make up the difference, make $600 million if the value goes up 20%, and leave some pension fund dumbasses holding the bag if the price goes down 20%.
Was the debt intended to help them grow?