My newly bought, first-hand game of Rummikub (from Philos) came with a die. However, the game doesn't seem to use a die. Is there any variant that uses a die, if so, what for?
There is a game called Dice Rummikub, which uses dice instead of tiles; while related, it is its own game and requires more than a single die.
Short of being mixed into the box accidentally, or tossed in as a convenient first-player randomizer (because "high tile goes first" is just so complicated :p), I can't think of any reason for its inclusion in the regular tile-based Rummikub set.
I could be misremembering (I couldn't find a source), but I think my old set of Rummikub might have included a die purely to decide who goes first.
I definitely remember buying a game that included a die that was only used to see who goes first. I remember only because it seemed so absurd as there are many other ways to decide who goes first.