It's a vampire thing. it's also an I-made-it up thing. We never see most Buffyverse vamps eat anything but blood, though we know they can, because in addition to Spike we've seen a villain-of-the-week eating Jiffy pop. What Spike eats canonically is blooming onion, hot chocolate with marshmellows, Weetabix in his blood, and alcohol, but I doubt that's a complete list, just what they happened to show. As far as I know, there's no canonical position on what it tastes like or why they would bother (or what happens to it afterward if it's not digested, but hey, it's magic! Like why all their blood doesn't pool in their feet and they can get erections.)

I decided on my own that this was an explanation that would hang together and be consistent, but I certainly don't expect all other Buffyverse players to stick to it if they have their own ideas of how it works. Spike is used to being odd.