The plugin directory is for standard Vim plugin scripts.

All files here ending in .vim will be sourced by Vim when it starts up.
Look in the file for hints on how it can be disabled without deleting it.

getscriptPlugin.vim get latest version of Vim scripts
gzip.vim edit compressed files
matchparen.vim highlight paren matching the one under the cursor
netrwPlugin.vim edit files over a network and browse (remote) directories
rrhelper.vim used for --remote-wait editing
spellfile.vim download a spellfile when it's missing
tarPlugin.vim edit (compressed) tar files
tohtml.vim convert a file with syntax highlighting to HTML
vimballPlugin.vim create and unpack .vba files
zipPlugin.vim edit zip archives

Note: the explorer.vim plugin is no longer here, the netrw.vim plugin has
taken over browsing directories (also for remote directories).