Okay so as a Vocaloid fan, I'll try to explain best as I can

First off, computer voices are just cool, and we just like how a lot of them sound. I love Kagamine Rin's voice for example, and still would even if she wasn't a computer program.

Second, the community surrounding Vocaloid is absolutely massive. If you like a real singer, they'll have a limit to how many songs they can release. But since this is an instrument, anyone can release a song with practically the same voice (practically since people usually tune it differently). There's music in tons of different genres, by tons of different artists, all sung by Miku.

Third, I'm assuming when you're talking about 'appeal' here you in some part mean the concerts. Well, on these concerts the band covers both original songs made specifically for the event by prominent Vocaloid producers, and band covers of existing popular songs. At least I think that's how that works, I've never been to one before. So there's not only hearing songs you like on there, but also meeting with like-minded people with the same interest as you. Also the animations for them are just so damn smooth

Four, a lot of us are just weebs

I'm bad at explaining but I hope that helped a bit at least :)