There was another factor at play in the OC, however, the game spawned in more enemies if you had a henchman, so you got slightly more xp for that too.
Much of NWN's combat is typically set up using
Encounters, which are configurable spawns that will, within the set boundaries, scale to the estimated challenge rating of the player's party. They're used widely from the OC to Swordflight, though to varying degrees, to control for non-linear progression, variable party size and co-op.
Encounters can be single-shot or repeatable, have a difficulty level, specify a minimum and maximum number of enemies spawned, and the list of creatures they can draw from, including whether any of them should be unique. For example, testing with a default Moderate Demon Encounter:
1) a solo level 1 Rogue will spawn 2 Succubi (the encounter's minimum);
2) a level 1 Rogue with a Summoned Dire Spider (Summon Creature IV) will still spawn 2 Succubi;
3) a level 1 Rogue with a single level 15 Cleric henchman will spawn 1 Bebilith, 1 Succubus and 1 Doom Knight or Vrock;
4) a level 1 Rogue with three level 15 Clerics will spawn 3 Bebiliths;
5) a level 1 Rogue with a Summoned Balor (Gate) will spawn 3 Bebiliths and 1 Balor;
6) a solo Fighter 14 / Blackguard 8 will also spawn 3 Bebiliths and 1 Balor (the encounter's maximum).