One of the biggest challenges for a small business is getting a handle on business finances. While "we're making more than we're spending" may be sufficient for a business that's just getting on its feet, at some point, you need to start looking for more detailed information on just what your expenditures are buying you.
Marketing has traditionally created a problem for calculating returns, because many traditional forms of advertising are "fire and forget" strategies. If you put up a string of billboards, you'll never definitively know whether those billboards are creating sales.
However, with inbound marketing, the calculations become a lot easier. Once you've been running an online campaign for a few months, or a year, you'll have the data to start really figuring out the Return on Investment (ROI) of your marketing spends!