Magicka sorcerer often use it as a small fortress, where they can stay in and be a bit protected against melee fighters. It deals nice damage and roots enemies stepping in, basically the magsorc stays either in the circle of mines or kites around the circle, so that the mines are always between the sorc and the enemy. A pretty good tactic is to stun your opponent in the very same moment they are either charging or rolling through your minefield, making them very vulnerable by eating the damage of all those mines and also spending stamina to break free. Even if they roll dodged through your mines and you get the stun off, they even might be rooted after cc breaking (when the root immunity wears off in that time). In a 1v1 situation, the atro ultimate serves this purpose very well.
It also helps out a bit, if you try to disengage or want to flee from a mass, since people need to go around them. Especially useful in crowded places, where going around them either takes very long or isnt even possible.
Another use of them is to place them in breaches in keep fights to root the very first line of enemies. Also there are many crowded places in keeps, where they provide a good opportunity to damage and root enemies (stairs, resources towers, in keeps near the upper flag, etc).
Keep in mind, that the skill is not the cheapest and it requires some skill to use it effectively.
In PvE that skill is not really used much.