(Holy shit fuck dammit I had a response typed to this but then I hit backspace and this somehow made my entire browser go back a page despite having text selected aaaugh)
Even outside of the cannibalism trait shared among Forsaken, there is evidence in-game of undead of all kinds being able to metabolize substances. Aside from the fact that many undead seem to favor mushrooms as an edible, the first that comes to my mind is the Alliance follower Delvar Ironfist, who does not neglect his Dwarven heritage in death and spends his time in your Garrison and at your side completely smashed. Since he’s of the blood school he’s probably in that higher tier of undead who are almost lifelike in function, so this probably has some impact on his ability to enjoy things (one of the many goofy books indicates that “alcohol is just not the same”, implying that this is not a universally shared experience). Thassarian also attempted to get drunk, though there was no follow-through on whether or not he successfully did so.
There’s also the inclusion of Corpulous, Acherus’ very own chef who even wrote his own book about mess hall etiquette! The book, like a lot of the books in Acherus, is obviously meant to be tongue-in-cheek but the fact that the Scourge felt it necessary to include a cook at all is still worth noting. Is it for the purpose of morale and control (give them food so they don’t turn on each other in a frenzy), or is it actually necessary to keep them “stronger”, as Corpulous writes? There is also mention of a ghoul being an excellent cook.
I think, like I answered with my post about breathing, it’s more a case of “can” rather than “need”. It’s unclear what benefit eating would hold for something that is already technically dead. As far as cannibalism goes the best I can speculate is that there’s some sort of “life energy” contained in fresh meat that helps stave off decay and encourages regeneration. That wouldn’t explain why Corpulous seems convinced that the more rotten the better, but maybe he’s just not a very good cook. Abominations aren’t terribly bright.
As with a lot of my answers, there’s a lot of case-by-case stuff. Some undead are missing entire chunks of their insides, so food would just fall out. Mine’s entire middle cavity is completely replaced with a beetle colony and he can’t eat at all. Others bear no significant difference from the living other than “a little pale and glowing eyes”, and the latter are probably lucky enough to be able to enjoy a good meal just fine!