The reason why I'd like to have these "units" is the same they got units in the army: to form kick ass groups that will have great chemistry and sense of awareness when they fight.
Why form these units?
-Cause you will learn the way your team mates fight and you can predict what they would do in certain situations.
-There is chain of command, everybody agrees to follow the leader and obey commands. Everybody knows what they are supposed to do.
Here is a list of my chars. They are all PvE chars, but almost every char has the equipment and skills to run every build worth running.
-Ernjo Perwo: Assassin, this char can spike, pressure, cap and move around quickly for repairs and stuff. Will get close and personal with casters.
-Esa Nahka: Ranger, can interrupt, spread conditions, deal AoE dmg and cap. Can run beast mastery and dagger builds.
-His Turdness: Ritualit, can heal, spike, spam spirits and even work as a minion master.
-Mr Ernjo: Monk, can smite, prot, heal, even spike. Very evasive with melee, don't mind being hexed with backfire Working on getting PnH, can run pretty much everything else.
-Rotten Juju: Necromancer, can MM, curse and degen. Not used as much as much in PvP, but can run most of the necro builds.
-Spol E Gona Pappa: Warrior, can run all the best war builds, I prefer hammer and spear, but can run axe and sword builds too. Working on getting sup vigor rune.
Toshi Death Note: Mesmer, can run every mesmer build, rly good at shut down and degen pressure, but can do interrupts. Maybe my best PvP character.