In Warcraft 3, you can hire Mercenaries from the Mercenary Camp. This camp exists on alot of classic maps. Nightelf is the race the hires the most Mercenaries since you use alot of ranged units and have limited (mobile) healing capabilities in the beginning.
Usually, u can hire 4 different of these Mercs –>
- Shadowpriest (Ranged unit with average damage, healing ability like a priest and abolish magic (dispell)
- Troll Berserker (Ranged unit with good damage)
- Mud Golem (Weak melee unit that can cast slow)
- Ogre Mauler (Strong melee unit, good tank)
Since Nightelfs use ranged units (Archers) in the beginning anyways, you can always just skip on 2 archers and get the Shadowpriest and the Forest Troll Berserker from the Merc Camp.
Mercs can be useful when u need additional forces in order
- To push your opponent
- Defend your base if the opponents army is bigger
- Break 50 supply heavily. Elf likes to save gold and wood on 50 supply and then start producing 2 or 3 more units from ancient of war and ancient of lore. If you also hire mercenaries at that point you can easily reach 70 supply within a matter of seconds even if u just had 50 supply). This often overwhelms the opponent.