The practical impact of who's granting the cleric's spells is...what exactly?

[Nellisir]

IMC, it matters because while having faith is enough to get low-level spells, someone has to be actually answering the phone to get higher-level spells. So a hermit or wise woman might get 1st & 2nd level spells from general piety, and a cultist of the Dead Hand might get 3rd-5th level spells, depending on what kind of non-deific extra-planar power is backing the cult, but you have to worship a real deity to get 6th & 7th level spells.

It also matters if your deity is imprisoned, dead, or otherwise disabled - which seems to have happened frequently in Greyhawk.

 

[Olgar Shiverstone]

It also matters if your deity is imprisoned, dead, or otherwise disabled - which seems to have happened frequently in Greyhawk.

And thus creates the Tharizdun/Zuggtmoy/Elder Elemental Eye bait-and-switch in Temple of Elemental Evil.