Because I love mashups. I'm sure most people on the internets know about
The Grey Album, but in a similar vein there's now
Jaydiohead, putting Jay-Z lyrics over Radiohead to surprisingly great effect.
There're other amusing combinations like Destiny's Child x Nirvana to create a little tune they call "
" But for my money, the full-length 'albums' people put together is where the real creativity shines, because it's not just a list o' songs but actual attempts to run a range of different tunes and it's a total kick.