PCB Design - Polygon pour remove islands or not?
Certainly not the only reason, but a big one is copper thieving. 

When manufacturing a board, it's easier to control consistency when the board has equal copper coverage.  It helps avoid over/under-etching, helps balance the board's rigidity, and so on.  If you had huge open areas without a copper pour, and let the manufacturer do copper theiving, they might add a pattern of dots or diamond shapes, not connected to any net, to add more copper to the board to balance it.

A reason that you might not want these unconnected pour islands, or the dots/diamonds added by the board house, could be for reasons like controlled impedance or EMI.  In this case, you might end up telling the board house to specifically not do copper thieving in certain areas of the board... or you might choose to have those pour islands be tied to ground. 

In this case, you'd be leaving the unconnected pours on there yourself, not necessarily telling the manufacturer to add more copper to your board... but it's the same idea.