The ideal molasses for use in horticulture is blackstrap molasses (unsulphered), which can come from sugar beets or cane. It is a by-product of sugar refining, and still contains sugars, but also has a high concentration of potassium and minerals including calcium, magnesium, iron, copper and sodium. Blackstrap contains a variety of carbohydrates (depending on whether it it is from beets or cane) but primarily contains sucrose and simple sugars fructose and glucose. Other carbohydrates are present to a lesser extent; raffinose (in beets), galactose, pectins, hemicellulose, etc.
Molasses also contains an array of organic acids, some amino acids and B vitamins (pantothenic acid, niacin, biotin).
A teaspoon to a tablespoon per gallon can be mixed into irrigation water and applied to soil during veg or flowering phases. The simple sugars are a quick source or energy for microbes living in the soil, bacteria in particular. Molasses is also used in actively aerated compost teas for the same purpose, to stimulate and feed beneficial microbes. These organisms assist the plant in various ways, e.g. by retaining and releasing nutrients in the rhizosphere, actively seeking out minerals or water for the plant, out-competing bad organisms and protecting the plant. Microbes themselves need minerals in order to thrive, which makes blackstrap a preferable carbohydrate source in teas and in soil.
You'll have best results with well-draining and airy soil mix, as both plant roots and microbes breath oxygen.
The benefit is indirect, unless the molasses happens to supply a mineral or trace element that is currently limiting the growth of your plants (e.g. K), in which case you could see a significant benefit.
