The key words in a sentence often include the subject, the predicate, the direct object, the indirect object, the subject complement. What all these elements have in common is that they are strictly required for any sentence to be considered grammatically correct. Also, when reading them you are able to understand the meaning of the sentence.
Adjectives, adverbs and prepositions frequently add information which is not necessary for a sentence to be complete. They either modify nouns or verbs. If you decided to remove them from the sentence, it would still make sense and its meaning would not be altered.
