Why it's needed to "source" some Vim plugins?

From autotag.vim:

install details

Simply source the file autoTag.vim from your .vimrc file. This utility will (obviously) only work when using vim that's been compiled with python support.

Is this needed because this is a Python plugin in vim, instead of a vimscript? Aren't plugins in .vim/plug开发者_StackOverflow中文版in loaded automatically?

This plugin only works when I source it. Is this behavior expected because I'm using pathogen?

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There is no difference: if you place it in .vim/plugin, you don't need to source it from somewhere else.

Addendum

As Randy Morris explains in the comments, with pathogen.vim's magic, the equivalent plugin path to put the script in would actually be .vim/bundle/autotag/plugin.