>>2049432
It adds a second stealth mechanic and third vision consideration. In most MMOs, stealth is a low skill cap ability that gives rogues a massive strength and information advantage. The veil adds additional skilled based mechanics to the paradigm. You have to consider the casting time, resource cost, and surrounding enemies when using it. When using it correctly, you'll gain quasi invulnerability, additional damage abilities, general freedom of movement.
That said, the veil is an overarching theme they've been working with since the beginning. As development continues, we'll likely see veil storms (unlikely at launch) that will alter the battlefields when too many players are zerging in a specific area. This helps thematically control realm imbalances.