It would depend on the context: "Sunlight is essential for photosynthesis", but "It is essential to remove your hat when addressing a lady". Can't explain, it just feels right.
Fatty acids that cannot be prepared within an organism by any known chemical pathways; and therefore must be obtained from the diet and hence are called essential fatty acids. And the two essential...
The phrase means that they are essential in the diet, because the body cannot produce them internally from other compounds. It should not be interpreted to mean that they are somehow more important...
essential amino acids are those that we cannot produce while non-essential amino acids are those that we can produce by ourselves by nitrogen containing compounds in our body.