Can you elaborate on the reason a writer uses style within a text?

If you think about it, you can learn a lot from how a person speaks, how they move their hands as they speak, the volume in which they speak, how close/far from you they stand, etc. The reader doesn't have that privilege when reading a text.
A determined author would use their style in their speech/text just as any other person would use their style when they speak or text. Style is something partly inherent in all of us, and it definitely shows part of who we are. In the case of written speech, style becomes much more important because the words on the paper are the only tools the author has to project who they are, their emotions, thoughts and tone through their work.