Skeletons, although fragile, have a high growth rate, and if you gather a large number of them, they will be excellent warriors. Furthermore, like fairies and peasants, they have a building that increases their growth rate. They are also easy to resurrect, and after a battle, if you have the necessary skill, they are reborn in proportional numbers.

Added 15 minutes later
Another addition: I mentioned earlier which creatures also have a high growth rate; these can, of course, be transformed into skeletons. And not just them. In general, I think that everyone has their own advantages. Skeletons - NUMBER Fairies - ASPEN (OR BEE) SWARM IMPS - STEAL THE ENEMY HERO'S MANA PEASANTS - CAN BE TRANSFORMED INTO HIGHER-TIER CREATURES THROUGH TRAINING, AND THEY ALSO PAY TAXES PROMPTLY. GNOMES - SORRY, I DON'T SEE ANY ADVANTAGES FOR THEM. BUT IN PRINCIPLE, THEY ARE TOUGH. ASSASSINS - ARCHERS, LEVEL 2 - ARCHERS WITH DEADLY POISON. LEVEL 3 - MELEE INFANTRY (INVISIBLE) - AN ACTIVATABLE ABILITY. GREMLINS - CAN CARRY A MAGE'S AMULET AND REPAIR MECHANICS. LEVEL 3 - SABOTEURS, CAN DEACTIVATE (RESET THE INITIATIVE SCALE DOWN) ENEMY MECHANICS. GOBLINS - FOR CYCLOPS - AMMUNITION AND HEALTH REGENERATION. FOR SHAMANS - A SOURCE OF MANA. WELL, I WROTE ABOUT THE HORDES.