We should keep in mind that we don't have access to all the available information the investors had. Previous successes by the founders, a kick-ass business model or solid traction could all be valid reasons for such an investment.
Our valuation of the company is based on partial information, and is likely less accurate than that of the investors.
If the app is useful, a premium version could be made, as well as a company branded version for an entire company, etc. HR is tough to sell to, but that doesn't mean impossible. It could be a nice corporate perk.