Bond Ratings. Companies pay rating agencies such as Moody’s and S&P to rate their bonds, and the costs can be substantial. However, companies are not required to have their bonds rated in the first place; doing so is strictly voluntary. Why so you think they do so?

Bond Ratings

Companies employ rating agencies such as Moody's and S&P to rate their bonds despite the substantial costs and their voluntariness because ratings by these agencies add a badge of honor to the bonds.  It gives investors some level of assurance that the bonds will be honored at maturity and that the pricing is right, given the company's credit risk.

Explanation:

Credit risk rating agencies assess the credit risk of a company or financial product as formal and credit-worthy benchmarks for investment decisions.  While companies pay huge costs to have these ratings conducted by the big three, including Moody's, S&P, and Fitch, the main value goes to the potential investors who require the information to decide whether to invest in the rated companies.