If the monsters would not move when they are in camp, it would make raids so much easier... but unfortunately there are a lot of factors that cause monsters not to stay on the spot. Some of them are clear - like when someone aggroes the monster, the monster will usually move towards the aggroer; the other factors are not so apparent.
One of these factors is push, which can generally be divided into melee push and caster push.
Every time a monster (or a player or a pet) is hit by a melee attack, the target of the attack will get pushed by a small amount in the direction of the attack. The amount does not depend on the amount that it hits for; each successfull melee attack (even if the attack does not cause any damage) generates the same amount of push. The amount is very small, but since melee hits happen very often and since a lot of people (and pets) are meleeing, these numbers add very fast.
Casters also generate push, usually with nuke spells: every time a monster is hit by a nuke, the monster is pushed by a small amount in the direction that the caster is facing. Usually, push generated by nuke spells is much smaller issue than melee push, because nukes happen much less often than melee swings, but it needs to be taken into account as well.
Special abilities - like some melee disciplines or knockback spells - generate a HUGE amount of push instantly; using these abilities should be avoided on raids at all costs, unless the raid leader asks for them. Usually the idea is to keep monsters on the same place, or almost the same place, during the whole fight - and it's difficult enough even without those special abilities.
The previous sections - especially Meleeing - already explained most of the reasons why it's a good idea to keep a monster on the same spot, or at least to control its movement in a predictable way. The other reasons, not related to meleeing, are apparent - we don't want the monster to be pushed towards possible adds, we don't want the monster to come out of spell range, and so on.
Root spells do not negate push, they will simply prevent the monster from moving on their own, so don't even think of using root to negate push. It won't help and it will only cause more problems as explained in one of the earlier chapters.