A good vocabulary increases successful communication. It makes it more efficient and sophisticated. Picking the right word cuts down on wordiness. It is thus economical. Things read more authoritatively and thereby it psychologically adds persuasive ethos to your writings. It adds precision and depth.
How, though, do we increase our vocabulary?
Without having a sufficient size, we don’t have much to draw from. My answer is to write down unfamiliar words in a notebook. I’m admittedly not always consistent, but I have begun to do this while reading. When I see a word I don’t know, I look it up, and then write it down. It doesn’t have to be a word I don’t know, actually, just something unfamiliar or unusual. Such a word sometimes gives the air of familiarity when an expert writer places it into a flawless context, yet alone it is actually unfamiliar or unusual. So look it up and write it down.
A thesaurus is a useful tool, though it’s inadequate. Like randomly searching a dictionary, a thesaurus unlikely will help you find a word that represents a complex idea you want to convey but which you cannot simplify to a specific word. That’s why you need to build your mind’s vocabulary.