It's a common excel trick in finance. For example, let say you have $0. If you borrow $1,000,000 at 5% interest, by end of year you'll be short $50k. That means you actually needed to borrow $1,050,000. But the extra $50K causes more interest ($2,500)... so you needed to borrow $1,052,500, which causes more interest... and so on.

Instead of doing some Excel Goal Seek or Solver or VBA macro, it's nice to let the excel "reactivity" handle it for you.