The world of hard rock is not a genre known for its subtlety. Whether it's through guitars, drums, or voice, every single instrument is known to be at full capacity from the minute the track starts playing. It's easy for artists to cut right to the chase, but there's a certain professionalism when it comes to introducing the main riff.
Regardless of which instrument takes the lead, these songs are masters at building up to the musical payoff. Rather than just hitting you over the head with the main riff, these songs take their time and build to some of the most satisfying riffs ever conceived by man. More than just announcing songs, these musical fragments act as a sort of welcome mat for the listener, as they get everyone used to the epic juggernaut of a song they're about to undertake.
It could be the beginning of a feel good song or the start of a new musical revolution, but all of these songs would not be where they are today if these little intros hadn't set things off in the right direction. Anyone can be in the right place at the right time when they write a great riff, but only the greatest musicians know how to really start their songs off right.