a shaman has 4 types of totems they can put down: earth, wind, fire and water.
there are several of each type so having 4 shaman to put down 4 different types of an earth totem can really stack the party buffs.
in BC the shaman could stack the totem of wrath ( which doesn't stack anymore ) which back then gave +3% hit and +3% crit rating ( x5 is 15% crit and 15% hit rating, you litterally couldn't miss and crit almost all the time or at least 40-45% of the time with talents and gear)
sometimes you want them all to put down the same kind of totem such as a healing stream totem x4 or x5 can be huge.
at level 80 my healing stream totems roughtly give about 200 health per tick so x4 is 800 per tick, x5 is 1000 per tick.
at level 80 my mana stream totems roughtly give about 30-35 mana per tick so x4 is 120+ per tick, x5 is 150+ per tick.
shaman can heal.
gounding totems can stop incomming spell.
tremor totems chan stop fear or fear bombs.
shaman can self-rez and res-others, you rarely have all 4 or all 5 shaman on the self-rez cooldown unless your having big trouble.
If you combine the team with a pally, he can rez as well.

are they the highest DPS, usually not
are they the highest Healers, usually not

but grouped together they are a pretty deadly combination.