These qualities make the Vigilance Guardian a great asset both in group PvE content as well as PvP, tearing through trash mobs or enemy players with ease while also knocking off huge quantities of health from bosses.
It also throws some highly effective AoE damage into the blend, making it a highly versatile class that can wrangle just about every kind of encounter, be it single-target or crowd control. Vigilance Guardians offer a blend of damage-over-time and burst damage abilities, paired with surprisingly high survivability for a DPS spec (anecdotally, we've managed to tank easier Flashpoints with a Vigilance Guardian). When it comes to the Guardian, the Focus discipline is widely considered to be one of the worst specs in the game both for PvE and PvP content, so if you want to play with a Guardian character in a DPS role, Vigilance is definitely the way to go.
While SWTOR fares pretty well in terms of balance, when you've got 8 classes, each with 2 advanced classes which each have 3 disciplines, a few of those whopping 48 specs are going to fall behind.