It starts with Deployement. My opponents know I'll be bringing my fanatics, so will think/doubt about their deployement or deploy a bit different.
Most people will use up resources trying to get the fanatics out. Resources wich else might go to other things.
The only problem with fanatics is that you can't charge an enemy unit, or you'll get a (big) chance of getting the hits yourself/having to out the trough your flank, in wich case they'll be whirling in your line.
Full Fanatics cost 75 points for a unit, but in the case of Gobbo's, Fanatics usually kill more then our WS2, S3 guys to. It's also the unit's answer to heavy armoured troops.
Like said, you don't really want to charge with a unit that has fanatics, wich makes animosity quite dangerous.
The good thing is, O&G have alot of different targets for Warmachine hunters/Redirectors. Fanatics, Manglers, Doom Divers, ... the opponent has to make a choice wich one of those he (tries to) kills.
Good thing about fanatics is, that 3/4th of their advantages even counts if you DON'T take them, if you're playing with closed lists.
Even just putting them on the table next to your dice/Doom Diver template is enough to scare your opponents (altough a bit tricky, and considered unsportive by some )