The value add to employers would be a standard assessment tool, reduced screening time for employees that can be eliminated quickly based on lack of skill, and using an assessment that is potentially better correlated with good employees than things like degree and school, or previous employer (decreasing both false positives from people who can't hack it despite their credentials, and decreasing false negatives by letting people with the skills demonstrate them).
For prospective employees, you get a single assessment that may open up a lot of jobs. Since the company doing the assessment isn't the one hiring you, they can tell you what you need to work on, even provide training opportunities.
Imagine someone telling you, "You aren't quite what we need, take these onlike courses and pass them and we will almost definitely be able to find someone to hire you."
And by acting as an intermediary between prospective employees and employers, the time cost of submitting resumes to employers that aren't well matched is decreased.
Further you might be partially eligible for a job, but lack specific skills, and they can ask if you are willing to take a temporary pay cut in return for on the job training as you work.
One of the things personally holding me back, is that you really need to be in school to get an internship. I already have a degree, just not in CS, and I'd love to be able to learn on the job at the intern level, instead of feeling stuck unless I go back for a master's degree.
I think there is a real opportunity, not just for such a company, but for a company that creates a standard or standards for assessment and licenses it/them to different companies.
Further, such assessments would be useful to assess not just individuals, but educational programs. Compare performance for people who graduate from different programs, including online training.