I'm assuming you're talking about derivational morphology: adding prefixes and suffixes to words to change their part of speech.
The answer is: because it gives you more words! Take the word "dependency" for example. This is a noun derived from the adjective "dependent", which is derived from the verb "depend". So learning the one word "depend" and its meaning, lets you create several other related words as well.
For another example, let's say I'd never seen the word "judgmentally" before. But I know the verb "judge", and I know that "-ment" turns a verb into a noun (the result of _____ing), and that "-al" turns a noun into an adjective (pertaining to _____), and that "-ly" turns an adjective into an adverb (in a _____ manner).
In short, derivational morphology lets you say more things, without memorizing too many more words.
Other interpretation: if you're talking about "verbing", where a word is treated as a different part of speech without changing its appearance at all, that's interesting morphologically because it changes the other morphemes you can use with that word. You wouldn't expect to be putting "-ing" on nouns, for example.