Consider the sentence: He looked up the word in a...

The particle functions as a preposition, introducing the object of the verb.

The sentence can be rephrased without altering its meaning to He looked the word in a dictionary, because the particle is optional and does not contribute to the verb's lexical meaning.

The verb retains its basic meaning, while the particle adds an optional semantic nuance.

The particle and the verb fuse into a single lexical item, whose meaning is not predictable from the parts.

Phrasal verbs can always replace single-word verbs without affecting style or context.