There is a short simple answer to this and a long detailed answer to this. We are going to go with the middle of the road for now. More details, the more we learn.
First and foremost, Full Spectrum has everything in the plant in the bottle....all the terpenes, flavonoids, cannabinoids, THC, CBD....all of it. All the good stuff. This is what we call the Entourage Effect. The Entourage Effect is when you get a synergistic relationship, meaning all the molecules are working together, including THC, the therapeutic benefits dramatically increase.
When you take a product labeled ISOLATE, the CBD has be ISOLATED from all the other superheroes. So, your product ONLY has CBD in it. When you can get the rainbow, why would anyone get the isolate? There are several reasons. Number one, drug testing isn't standardized yet. If you get drug tested at work or school for THC, you want to take the isolate. This is not because you are taking something illegal. This is because drug testing has not been standardized yet. Example. When you get in a car wreck and they suspect you are drunk, they will take a Blood/Alcohol test where they measure the ratio of blood to alcohol in your blood. There is a "legal limit" and even if you have had a drink, you can be tested and cleared by a blood/alcohol test. However, we don't yet have a standard THC test. So, you could be confronted with a positive/negative test or a ratio test. Either one you take you are legal, but a ratio test, if it happens to test positive (we are talking like testing positive if you eat a poppy seed muffin), your ratio will be clear from the start, and you will be cleared. However, if you were to take a drug test that simply tests for the presence of THC ( +/-) you MIGHT be positive because if you take Full Spectrum you are taking up to .3% THC. You are still legal, but until the testing is standardized, it might become a headache for you to deal with. I tell everyone this because I would NEVER want to be the reason that anyone was to lose their job or their place on a team. That would be a decision you will need to make on your own.
All this to say, one of my best friends is a nurse and will not touch the Full Spectrum. She takes CBD Isolate and has many positive results. You can see by the picture (visual learner here) what you gain by taking Full-Spectrum. All the pretty colors (80%) of the wheel make up the different terpenes, flavonoids, fatty acids and approximately 20% of the plant is comprised of a variety of cannabinoids (CBG, CBN, etc). Only ONE of those little grey segments represents CBD alone. So, when you take an isolate, you are getting simply that one CBD segment vs the entire color wheel when you take Full Spectrum. While CBD is amazing, the Entourage Effect, using all the components of the plant is mind boggling in its effectiveness in providing benefits.
We will get to the different terpenes later, as well as the flavonoids and even the other CB's. CBD and THC are but TWO cannabinoids found in the hemp plant. They happen to be the most studied and understood, but they are but the tip of the ice berg for what we can gain from the molecules in a hemp plant.