First Aid is one of the less used secondary skills in the game. At Expert level it only heals 100 health points, which means that all creatures up to the fifth level and most of the sixth level creatures can be fully healed . However, considering that the healing is only given to a single monster, the impact of the First Aid Tent is almost always negligible. Though First Aid isn't useless, most other skills in the game are generally more useful.
The most common situation where First Aid can be worth getting is if the player manages to get one or more level 6 or 7 creatures early on in the game (e.g. Refugee Camp), as these units can receive the full 1-100 health provided by this skill. Furthermore, it should be noted that unlike most other skills in the game, the effect from First Aid can never run out of mana or ammunition. As a result, it can be very effective during long battles in the early game where the player has at least one level 6 or 7 unit, because as long as the First Aid tent survives and the high leveled units do not take more than 100 damage per turn, they are effectively made imperishable by First Aid. However, it can still be more advantageous to choose another secondary skill than First Aid even in these situations, as the First Aid unlike most other skills will become less useful the longer the game goes on, as the 1-100 health provided by it becomes less and less substantial.