You may be baffled because this is a very complex subject. Much of it goes over the heads of many stoner growers. I contunue to learn every day. That is fine. You can grow a good plant using just Mega Crop. You can grow better plants with just a little tweaking. A main reason for having supplements to the base fertilizer is driven by chemistry. If some nutrients in concentration are mixed into the base they bind to other nutrients making both unavailable to the plant (lock-out). So they cannot come in the base but are very useful to the plant and a well balanced fertilizer.
For me growing my meds is a main activity as I am retired from work to watch the grass grow. The desire to do the best I can drives the search for knowledge in the use of nutrients and methods of growing.
If you are in soil (especially organic) there is much to learn at first and then it falls into a routine. The well prepared soil holds and provides the different needed nutrients that are available when needed by the plant. Supplements in this case play a lessor role. If you grow Hydro this is a whole different practice. You must provide the nutrients needed by the plant when needed. Cannabis (as well as most plants) need different nutrients at different stages of growth. This is the reason for so many Veg, Flower and additives in the market.
With the new 2 part Mega Crop You can dial in the NPK ratios to match the stage of growth the plant is in.
Some additives that I find useful that may not be in many base nutrients (not in the new 2 part MC) are Kelp, Humic and Fulvic Acids, B1, Silicon, Yucca Powder and Botinicare HydroGuard. I also supplement with some NPK Industries RAW products Potassium, Phosphorus, Nitrogen when the plants tell me they need it.
If I could only have one fertilizer it would be the new MC one part.