Epic Rock is an alternative label for Symphonic Metal and similarly grand-sounding Rock Music for North America and elsewhere that Metal is defined by a complete absence of pop elements.
There are two ways to think about this tag:
1. Music that is "epic" in it's rock-ishness.
2. Music that sounds "epic" in addition to being Rock.
Number 1 adds no new meaning, being nothing but a statement of personal preference. Number 2 is a much-needed term for a kind of music that has no other suitable name in North America and is rapidly increasing in global popularity.
For potential grammarian detractors, yes of course "epic" is traditionally a noun referring to a genre of literature, but it has become an adjective to describe anything which has a similarly grandiose quality to that literature. .
