The answer is that Teleport is very good, because it's useful, slightly undercosted, and versatile.
Most attack/incantation spells that do damage offer a return of ~1 damage per mana. By teleporting an otherwise safe creature, or a creature otherwise unable to attack, damage is gained at the rate of the creature's attack ability. When this happens, it is often to a golem, hydra, or bear, which all hit for 6-9 dice of damage on a slow attack. @1 space that can be a 2-3:1 ratio, and @2 spaces can be 1-1.5:1, still very good. This can get even more ridiculous when a single target is moved to a space with multiple creatures that couldn't get to it. Now we multiply our ratio by the number of creatures. A golem or plant "pit" waiting can easily dish out 30+ damage in a single turn.
On the flip side, healing is worth about 1 mana per healing die. When a slow creature is teleported away it loses ~1 attack per square moved, and a regular creature without fast loses ~1 attack per 2 squares moved. Spending 6 mana against something like a Silverclaw Grizzly prevents 5-7 dice of damage. Now, factoring in armor or other defenses, this is only okay for healing. However, keep in mind this is one of many modes to choose from! Having a damage prevention choice is solid, and we're only paying a small premium over Block! (And if we teleport a Slow creature, forget it. We've hit mana efficiency gold!)
Finally, there are spells like Tanglevine and Quicksand that are broken by a teleport. This can be fine when it's just 3 mana trading for 5 and 2L mana, but really ridiculous when they can also derive another advantage out of the teleport in addition to breaking the conjuration.
Also, sometimes moving is just useful.
Teleport's by no means essential. It just offers some really powerful returns on your mana, which can be hard to pass up.