It's not, to my mind, an actual corporate handbook -- it's missing boring important things like "here's our password and security policy, please remember we have people's super-personal health data in our apps, so it's really important". I guess that's probably in the company Wiki or similar.
However, this whole thing started out as a Onboarding Handbook for new team members. Then however we thought it's also a great book to share with the world. So we decided to remove certain items and have those in the internal wiki only.
1) A recruitment tool
2) Marketing
Recruitment and marketing are definitely a good side effect, but as @znq mentioned earlier, we built the guide in the first place for onboarding our own team members, then we thought sharing it publicly would benefit the community.