The .git folder is literally the git repository; all the indexes, history, structure of branches and tags, and the changes over time to the files themselves (broken up into chunks and assigned pseudo-random filenames in the form of a hash of the content it represents).
Simply the presence or lack of this subfolder identifies the current folder as a “git repository”. It’s one with it, and not one without it.
You should neither delete this or edit it directly, as doing so can mean an unrecoverable loss of that history unless there is another copy of the same git repo elsewhere.
Any edits to the .git folder contents should be done using the git command line tool or a UI like sourcetree. It’s rare to do otherwise, and even then you typically only alter a simple plain-text config file (which by and large the tools can process), and never get into the guts of the repo structure itself.
Just so you know, SourceTree is a GUI that drives the “git” command line under the hood. Even SourceTree by and large is clueless about how a .git folder is structured. Instead it relies upon issuing git commands behind the scenes and then reporting the results it outputs back to you, in a prettier format than the git command is generally capable of doing.