Is there a point to eating doughnuts?
Solution 1:
It's very likely to be just an Easter-Egg, I can't find any actual use for them.
The Easter Egg refers to The Simpsons Doom where donuts were used as health packs, I cannot guarantee you that it's definetly pointless to have it (other than for the sake of completionism), but it's very likely that it's just that, an easter egg referring for another Mod of Doom: The Simpsons Doom
This thread has a discussion of the (then) upcoming Doom mod, which is the first mention I could find of donuts.