Okay here is all you need to know in a tl;dr.
Microorganisms: fight pests, assimilate nutrients for your plants improving uptake by breaking down organic material, results in healthier roots, more roots and hence bigger plants.
Enzymes: Makes the decomposing process of organic matter faster. (Like organic ammendments, organic liquids or old roots). Helps keep the soil clean since youre taking whats in there before adding more ferts. They also convert accumulated salts in the roots from nutrients in food, keep the roots clear aswell for better uptake.
Micronutrients: Too many for a tl;dr. You use them when you want a little extra boost or just to ensure the health of your plant, youre not gonna get huge gains with many of them but with others you might. Silica for example makes your plant way stronger and thicker, its present in your medium already most likely (unless youre using coco) but adding an extra is very noticeable. Carbohydrates are crucial for sticky smelly flowers. Iron, Manganese, Zinc, etc are already present if youre using tap water or even bottled water (ammounts will be different, a ppm meter can give you an idea on how many minerals the water has), but Im not sure about RO water, thats when maybe buying those can make a difference. Many base ferts also include them. Calcium and Magnessium also come in tap water, but if your base fert doesnt have them and youre using RO water its advisable to use
calmag, if your base ferts have calcium and magnessium then only if any sign of deficiency appears. Calcium is crucial as a building block for the plant new material. Magnessium is crucial for photosyntesis and it also helps with salt build up in the soil.
Did I forget anything? Do you need more info? If so ask a question and I will try to answer if its inside my knowdledge and I will try to leave behind the science behind it and things that might confuse people.