It should be noted that while anyone can submit a BIP, they are in fact formal design documents. They not only outline the proposed change and rationale, but also the technical details of the change or feature to be added to the Bitcoin network. It is the default standard used to keep the entire community informed of ideas for improving the Bitcoin network, and how those changes could be implemented.
There is a very good reason for BIPs within the Bitcoin ecosystem, and that is because there is no centralized, formal decision making structure for the Bitcoin network. Without another clear mechanism or authorizations to make changes, the BIP provides a way for users to suggest potential changes, features and improvements that may or may not eventually get implemented.
There are several kinds of BIPs that vary depending on what they cover. Informational BIPs are for general guidelines and design issues, Process BIPs are for changes to the existing network process, and Standard Track BIPS are for technical changes.