Though players of Vanguard will be able to spend their game time isolated from other players if they choose, the most rewarding experiences will be had in groups of players that learn to work together. We believe strongly that grouping is one of the cornerstones behind community building, especially pick-up groups, and we are working on a variety of things to make it easier to pick up and maintain groups, and we are striving to create a sort of interdependence that will naturally fulfill itself through grouping.
The most obvious way for us to encourage player-to-player interaction is by creating classes that fill roles. When players filling different roles group together, they are far more powerful than a solo player or a group of players who all fill the same role.
Source: Official FAQ