When fighting bosses, it is advantageous to keep some of this ammo in the ammo slots so you can switch between these and your main ammo to inflict Ichor and increase damage dealt with your main weapon. It is not recommended to use this ammo type as a main, since other types (most notably Crystal Bullets) can inflict significantly more damage with the Ichor debuff applied.
Ichor Bullets reflected by Mimics in Expert Mode do not inflict the Ichor debuff on the player.
In a PvP scenario, the damage increase from Ichor actually changes depending on the difficulty: on Journey/Classic it will increase the effective damage by 10, 15 on Expert, and 20 on Master mode. This is due to how the effectiveness of defense changes with the difficulty, getting stronger as it gets harder.