Here's why:
* Suffixes are word parts that are added to the end of a word to change its meaning or grammatical function.
* "-ward" is used to form adverbs of direction, meaning "in the direction of" or "toward".
For example:
* back (adjective) + -ward = backward (adverb)
* home (noun) + -ward = homeward (adverb)
* up (preposition) + -ward = upward (adverb)
So, while "-ward" itself isn't a part of speech, it contributes to the creation of adverbs.