Sugar in a baked good is doing several structural jobs at once, and sweetness is only the most obvious. This is why reduced-sugar versions rarely behave like the original.
Sugar holds water away from the flour
Sugar dissolves readily and binds water molecules to itself, which means less water is available to hydrate flour and develop gluten.
That competition keeps baked goods tender, since a well-hydrated flour network is exactly what makes a cake tough rather than soft.
It also slows staling, because bound water leaves the crumb more slowly than free water does, which is why sweeter cakes keep longer.
It controls when things set and spread
Sugar raises the temperature at which egg proteins coagulate and starch gelatinises, delaying the point at which a batter stops flowing and sets.
In a biscuit that delay is the whole reason for spread, since the dough remains fluid for longer and flows outward before the structure locks.
Reduce the sugar and the same dough sets earlier, holding its shape and producing a thicker, cakier result that people often mistake for a mixing error.
Browning depends on sugar, not heat alone
Colour and roasted flavour in baking come from caramelisation of sugars and from reactions between sugars and proteins at oven temperatures.
Without sufficient sugar those reactions run slowly, producing a pale surface and a noticeably flatter flavour even when the item is fully cooked.
Which is why low-sugar baking often looks underdone at the point it is actually finished, and why extending the bake dries it out rather than colouring it.
Aeration in creamed mixtures is mechanical
Beating butter with sugar works because the sugar crystals have sharp edges that cut air pockets into the fat, and those pockets become the crumb.
Liquid sweeteners cannot do this, since there are no crystals to do the cutting, so a creamed batter made with syrup rises less and is denser.
Crystal size matters too, which is why fine sugar and coarse sugar produce visibly different crumb in an otherwise identical recipe.
Substitutes replace one function at a time
Intense sweeteners deliver sweetness in tiny quantities and therefore contribute none of the bulk, moisture retention, browning or aeration that sugar was providing.
Sugar alcohols and bulking agents restore volume and some moisture behaviour, but they brown differently and can alter texture in their own ways.
Successful reduced-sugar baking usually combines several ingredients to cover the separate jobs, which is why the recipes rarely look like simple swaps.