Rails - Jul 2011 - Gemfile, the asset pipeline: Why have the group :assets in your Gemfile?

Heroku, as shown here - 
http://devcenter.heroku.com/articles/rails31_heroku_cedar - shows a Gemfile 
in which the following exists:

# Asset template engines
group :assets do
  gem ''sass-rails'', "~> 3.1.0.rc"
  gem ''coffee-script''
  gem ''uglifier''
end

I don''t understand why these gems are grouped.  Why not omit the group 
:assets do ... end? What does this do, exactly?

I''m asking because something really peculiar happened: I get the
"Sass
Syntax Error: File to import not found or unreadable..." when the imports *
definitely* should work. When I remove the group :assets, run "bundle 
install", and then check rails server, the imports work fine again. And 
here''s the oddest of all: when I revert my Gemfile back to having the
group
:assets as above, and then run rails server - the imports *still* work 
although my Gemfile was back to what it was when the imports *didn''t*
work.
*Thoughts appreciated.*

(Btw, I use Rails 3.1rc4, and I''m trying to deploy to Heroku asap)

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You don''t need this gems in production

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