I think the workplace/company review aspect is how they market themselves, but really that’s just a small part of it. Companies are their customers, people interested in jobs are their product, so naturally they need to get as many people as possible to give them their personal information.

Edit: One of the “services” they provide is the ability to add your resume. Presumably that would provide some kind of job matching benefit, but I’m not familiar with it. But, that would be one source of users’ info.