Soul Balm generates 5 EP on use. You do not lose any excess EP that would go over your maximum EP; instead, you will temporarily have more EP than your maximum. EP will resume regenerating as normal once your EP drops below your maximum.
In addition to restoring 5 EP, during Unite and Fight events, Soul Balm can be used to grant a crew-wide 1% ATK or HP buff that stacks up to 30%. This buff lasts until 05:00 JST. In other words, a crew can spend 60 Soul Balms per day to maintain the maximum buff. This is an important bonus to have when aiming to qualify for the finals.
Soul Balms are rather rare, only regularly obtainable with Coop MP, which takes a very long time to reset. Events may have 1-2 of them as rewards, but overall it is very hard to accumulate many Soul Balms.