Funnily enough, sugar is typically found alongside most grain based foods. It's in breads, cupcakes, etc etc. Ruling out grains consequently get's rid of a large source of sugar. The two are pretty intertwined.
https://www.healthline.com/nutrition/sucrose-glucose-fructos...
Show me the chemistry. Show me how sugar in a raisin is somehow significantly fewer net calories of glucose than just eating the equivalent in refined cane sugar or wheat flour. Sure, it takes a small amount of energy to reduce other sugars to glucose, but I can't see how it can be enough to make a difference in a weight loss diet.
Eat white sugar you will quickly get an energy boost but whatever cannot be used will be stored.
Eat a complex carb and it requires more steps to breakdown. That allows for a slower release of energy and less of a need to store.
The digestive system is a pipeline of processes.