[Radek]

Donations seem to be the only way of getting Cure Potions early in the game. The potions are as rare as artifacts. In fact, they are seen more rarely than any artifact. I have seen very few ones late in the game (when you do not need them) but really very few ones.
Therefore, the potions belong to the most valuable items in the game. Collect them, use them only if you really must, and inherit them from game to game. They can solve problems with a diseased character, especially, in the beginning of the game.
Other potions from the temple are less valuable. You can also get
- Stone to Flesh Potion
- Mana Max Potion
- Restore Health Potion

As it has been said, donations train your PRE. Summary:
- you can train PRE only until you reach 18.
- you do not get any potion when you try to donate with PRE 18.
The PRE stat is meaningful only for companies. The character with higher PRE is attacked more frequently(?!??!? - you are punished for being nice, I would expect the contrary: the character with the lowest PRE is attacked more frequently). On the other hand, the actual value of PRE does not matter. The monsters will behave the same way when your whole company has PRE 3 as when your whole company has PRE 24. For a solo character, the PRE stat seems to have no meaning at all.
In the other words, PRE is rather useless stat. The only reward for donating in the temple is the occasional Cure Potion.