Simple syrup is treated as a single ingredient and comes in at least two common strengths. Substituting one for the other changes the sugar in a drink by a wide margin, and it changes the texture as well.
The two standard ratios are not interchangeable
A one-to-one syrup dissolves equal weights of sugar and water. A rich syrup uses twice as much sugar as water.
Per measure poured, the rich syrup delivers substantially more sugar, so a recipe written for one and made with the other will be noticeably out of balance.
Recipes rarely specify which is intended, which is the single most common reason a well-followed drink comes out too sweet or too thin.
Weight and volume give different answers
Sugar measured by volume varies with how it is packed, while measured by weight it does not.
A syrup made by volume is therefore inconsistent between batches, which shows up as drinks that differ without any change to the recipe.
Making syrup by weight takes no more effort and removes the variability entirely, which is why bars work that way.
Rich syrup changes texture as well as sweetness
Higher sugar concentration raises viscosity, and that thickness is perceived directly as body in the finished drink.
A drink built with rich syrup feels fuller at the same measured sweetness, which is why it suits spirit-forward drinks that would otherwise seem thin.
The same viscosity slows the drink across the palate, which lengthens the finish and softens the impression of acidity.
Concentration decides how long it keeps
Sugar preserves by binding water so that microbes cannot use it, and a rich syrup binds far more of it than a one-to-one syrup does.
A one-to-one syrup kept in a refrigerator lasts a matter of weeks, while a rich syrup lasts considerably longer under the same conditions.
Heating during preparation is not required for dissolution but does reduce the organisms present at the start, which extends life further.
Other sugars are not drop-in replacements
Demerara and other unrefined sugars bring molasses notes and slightly different sweetness intensity, so they change flavour along with sweetness.
Honey and agave carry their own water content and their own acidity, and both are usually diluted into a syrup before use so they can be measured consistently.
Once the sweetener carries flavour, it stops being an adjustable variable and becomes an ingredient the rest of the recipe has to accommodate.