I don’t think “essential” properly defines the semantic function of preposed adjectives. I’d say that preposed adjectives tend to be more subjective while postposed adjectives tend to be more objective and sometimes even restrictive. One of the best examples is “grande”, which as you probably know is abridged before nouns in the singular:
- gran hombre can be translated as “great man”, while “hombre grande” means “big man”.
This difference also involves the comparative and superlative:
- mayor problema can be translated as “main problem” (with mayor reflecting degree of importance), while hermano mayor means “elder brother” (where mayor refers to age).
We can say that assigning degrees of importance is highly subjective, while describing somebody’s age is quite objective.
A similar case is that of alto. When preposed it indicates hierarchy or intensity; when postposed, it denotes size. Thus, alto comisionado is "high commissioner", while "comisionado alto" is translated as "tall commissioner".
There are adjectives which will be similar, showing little or no difference when translated into English, whether they are preposed or postposed, for example:
altas temperaturas vs. temperaturas altas, where the choice will only be based on stylistic reasons.
principal personaje vs. personaje principal (“main character”)
We can also find preposed adjectives that denote size and merely form a set phrase or lexical unit, as is the case with alta montaña.
However, preposition will generally sound more subjective (e.g. "principal personaje" may sound like the main character in my opinion), while postposition will render a more objective phrase ("personaje principal" is closer in meaning to the main character in everyone’s opinion, or according to most readers, spectators or critics).
Finally, unlike preposed adjectives, postposed ones can be restrictive, meaning “the one/s that is/are like that” (rather than any others). Thus, el hombre valiente (as opposed to el hombre cobarde) refers to a particular kind of men, or to an individual man (the one who is brave).