All mages (except Morrigan) automatically start with this spell.
Although it is fairly weak compared to most damaging spells, its quick cooldown time and low mana cost, combined with the fact that it will often be stronger than a basic attack, make it a useful if not overly potent spell.
The long range, in some cases, allows the character to stay outside of the enemies perception range and fire at them without their attacking back.
The projectile from an Arcane Bolt cast as a spell will go through obstacles like a pile of crates or a mound of rocks (projectiles that are stopped by such obstacles trigger a "Blocked" floating text notification). Being within range and having line of sight to the target are still required.