Just to risk a quick detour back to the banana...
If you have a banana split- a scoop of ice cream, a banana, and a squirt of cream- you eat it with a spoon, savour every mouthful.
If you blend the same stuff it's a milkshake.
SW would say that after eating an indulgent banana split you'll feel treated, indulged, full. After eating a milkshake, less so. The calories are the same, the level of satisfaction is not, so you go on to eat something else.
If you eat a cup full of berries, a chopped apple and a sliced banana, that's a big bowl of fruit salad. It will take a lot of chewing and a while to eat. Blend it into a smoothie and it's a drink you can have on the go. Again, potentially less satisfying.
Slimming world warns against 'tweaks'- roasting chickpeas and eating them like peanuts, for example. Basically you take a low density, satisfying food and turn it into something you can quickly eat a lot of- undercuts it's credentials as a free food.