Why do I get the error in my acceptance test? - ruby-on-rails

Question

Tell me please,why does it happen?
I can't understand, if I write:
feature "Article Creation" do
#here i write (:all)
before(:all) do
sign_up_helper
end
I get the error:
Article Creation allows user to visit to creating article page
Failure/Error: fill_in :article_title, :with => 'test_title'
Capybara::ElementNotFound:
Unable to find field :article_title
or
1) Article Creation allows user to visit to article page
Failure/Error: expect(page).to have_content I18n.t('articles.articles_new')
expected to find text "New Article:" in "Toggle navigation Blog Rails New Contacts Sign in --- !ruby/hash:ActionController::Parameters controller: devise/sessions action: new {\"controller\"=>\"devise/sessions\", \"action\"=>\"new\"} nil You need to sign in or sign up before continuing. Sign in: Email Password Remember me Sign up Forgot your password?"
but, if I write:
feature "Article Creation" do
#here i write(:each)
before(:each) do
sign_up_helper
end
It's Ok. All tests works. My question -WHY?
This is my test:
*#before all test visitor signs up
#here I've changed :all and :each*
feature "Article Creation" do
before(:all) do
sign_up_helper
end
scenario "allows user to visit to article page" do
visit new_article_path
expect(page).to have_content I18n.t('articles.articles_new')
end
scenario "allows user to visit to created article page" do
visit new_article_path
fill_in :article_title, :with => 'test_title'
fill_in :article_text, :with => 'example_text'
click_button 'Save Article'
expect(page).to have_content 'example_text'
end
This is sign_up_helper method:
#spec/support/session_helper.rb
def sign_up_helper
visit new_user_registration_path
fill_in :user_email, :with => 'user#example.com'
fill_in :user_username, :with => 'mike'
fill_in :user_password, :with => 'secure123!##'
fill_in :user_password_confirmation, :with => 'secure123!##'
click_button 'Sign up'
end
This is html form:
<p>
<label for="article_title">Title</label><br/>
<input type="text" name="article[title]" id="article_title" />
</p>
<p>
<label for="article_text">Text</label><br/>
<textarea name="article[text]" id="article_text">
</textarea>
</p>

Answer 1

Environment for each test is set anew, I think. New session, cookies, etc. In many cases, even brand new users are generated. So one "global" login is not possible.
Even if it were possible, it would still be a problem, as it introduces spec order dependency which is bad. Imagine that one of your specs logs user out. Then each subsequent spec would fail, because user is not logged in anymore.
To prevent this, make sure that each spec sets its own environment as it needs it (user logins, method stubs, etc.), without relying on side-effects from previously executed specs (which may or may not persist).