These spells lower the abilities of enemy mobs. Largo's is a good example as it not only lower's your opponents armor class but also their attack speed. Perhaps the most sought after line of spells by bards in groups however are their downtime reducing spells. Bards have different spells which can increase themselves and their party members stamina, hit point and mana regeneration speeds. These makes bards invaluable additions to a party.
Bards also possess party utility spells.
These spells are very useful for a party as they can effect everyone at once. They are require careful attention on the bards part as people can easily move out of range and get into trouble. It would be a bad thing for the party if they relied on a bard for water breathing and he went linkdead or died while they were deep in Kedge Keep for example. This spell is useful if your puller brings back more than you can handle. If your party concentrates on attacking a single monster you might be able to soothe all the rest and prevent them from attacking.