The more gold you pay to a priest, the stronger his god becomes. Newly generated priests of that god could get better equipment, or altars of that god might be harder to convert. In Moloch’s case, Gehennom gets slightly harder if you pour a lot of gold into his temple. This applies with money donated to your own god as well, but the effects are naturally less visible since you won’t go around fighting your own priests or converting your own altars.