The goal of dot-config is to promote standardization of .config folder for storing project specific tool configurations.
By using a subfolder you will help decluttering project root folder and isolating configuration from other project files.
Use of .config is based on long standing XDG standard which promoted the use of ~/.config for keeping user settings. We only extended the concept to project repositories.
Q&A
Do we really need yet another standard?
Dot-config is not a new standard, is just an extension of XDG one, one that covers for projects directories in addition to user home directories.