My short answer would be, for the same reason that fast food combo meals include a sweet soft drink. Savory, salty and sweet are a great combination, and the marketing folks know it.
My specific reasons are:
- Balances the salt
- Balances the acridity and bitterness of the smoke.
- Bark formation
- Helps denature (tenderize) the protein
- Hits one more of the five tastes that the tongue detects (pungent, sweet, sour, bitter, and salty). The more tastebuds you tickle, the better.