The idea of Berserk mode is to have the homunculus act differently under certain conditions - specifically, it can be set to ignore minimum SP settings for attack skills, or always use skills, even if it normally would not.
Homun enters Berserk mode under three conditions:
If UseBerserkMobbed is set, if there are more than this many monsters attacking homun. Note that the Weight tactic is applied here (like almost everywhere else where monsters are counted), so you can make some monsters count for more or less than one monster. In any case, the homun will leave berserk mode when there are fewer than the specified number of monsters attacking.
If UseBerzerkAttack and/or UseBerserkSkill is set, it will enter berserk mode when commanded to attack or use a skill on a target. In this case, it will leave berserk mode when that one target has been killed. Useful while actively playing with homun out, to sic your homun on something.
The use case I imagined for UseBerserkMobbed was:
Say you occasionally get mobbed while AFKing, but homun usually isn't killing very hard. Set a high minimum SP, so it normally holds most of it's SP in reserve, and set UseBerserkMobbed and BerserkIgnoreMinSP, so when you do get mobbed, the homun will spam skills (using that big reserve of SP it saved up).