It gives like +1 faith. I think what
@Oberinspektor Derrick said and means is "saved faith value" when purchasing a great person using faith.
That's what I meant, yeah.
It gives a nice faith discount for sure, but the faith I do spend on GP is rather small overall (I only go for a select few, and often I can get them for free anyway), and either way the discount tends to come later in the game where the lack of faith is no longer so pronounced as I'm generally well stocked on faith income by then.
Overall I find the future saved faith to be an often harsh price to pay for diverting my production towards the Oracle over something else early on.
Edit: In a typical game with high faith income, I'll generally spend about 80-90% of my faith on civilian units (if Monumentality), Campus buildings (if going science with a Jesuit Education religion), units from the Grand Master's Chapel and (if cultural) every last penny on Naturalists and Rock Bands.
The remaining 10-20% perhaps might go into GPs, but overall that translates to very little faith.
If we assume that I would spend 100% of my faith on GPs, sure, that's like building a wonder that increases my faith by 25%, but when considering what I actually spend faith on, it turns out to be more like a 2,5 - 5% "extra faith" over the course of the game. Which is overall pretty bad for how I tend to spend my faith.