Reuse library is basically a repository for various function and classes that can be used. One of the main benefits is that it has a number of source code examples for these functions present. These examples are quite useful as these could be used as a base code and you don’t have to start from scratch. I don’t think that it is used for documentation of internal functions, although it varies from user to user and can be used for keeping the documentation.
There are no regular updates scheduled with various releases.
Menu Path for transaction SE83 −
SAP Menu→Tools→ABAP Workbench→Overview→Reuse Library
Transaction Description: Reuse Library
SAP Module ID − BC
SAP Module Description: Basis Component