When your organization is small, it likely knew a lot of information implicitly. People worked on the same page. However, as it grew so did the needs for recording information.
Tribal knowledge is a concept in which information is implicitly known and shared. It works ok in small organizations but starts to break down as it grows. People stop knowing the information as it stops coming up in conversation.
Handbooks are a way to document what is most important to your organization and keep an information hierarchy that makes sense to you. This developer productivity handbook is an example of this sort of system.
Each handbook is different and is unique to the organization. Topics vary, but often include things like:
Whatever your handhook develops into, you can be assured that the topics are helping by tracking what pages are viewed, links are clicked, and other metric collections. From this you can create an appropriate information architecture and knowledge management schema.