Why do I need to use concat even though I use <%= %>?

gistfile1.rb

# Partial: /views/songs/_song_info.html.erb

<div class="song_info">
  <div class="tags">
    <h4>Tags:</h4>
    <p><%= render_tags @song %></p>
  </div>
</div>


# Helper Method: /helpers/songs_helper.rb

def render_tags(song)
  if song.tags.any?
    song.tags.each { |tag| concat content_tag :span, tag.name, class: "tag" }
  else
    # Why do I need to use 'concat' here?
    concat content_tag :span, "This song has no tags! Add some!", class: "no-tags"
  end
end