My game of Rummikub came with a die. Why could that be?
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.