Writing simply helps us recall what we've read, observed, or discovered.
Writing helps us see patterns in the information we've encountered. Writing helps us explore intricate relationships, deconstruct complex ideas, and discover where researchers disagree. Writing may also expose areas where our understanding of a subject is incomplete. For this reason, many people find it helpful to write early in the information discovery process rather than waiting until the search process is nearly complete.
Writing is a good way to evaluate a person's thinking (even your own). Writing will help you understand gaps or inconsistencies in your argument. Writing can also help you eliminate ideas that aren't relevant to your purpose or are distracting from your thesis.
For many of us whose careers require us to devote a considerable amount of time to written communication, we know that writing is an important professional skill. Further, writing can be a way for us to help our audience learn and increase their understanding of the ideas we are exploring. If by writing we uncover areas where we have cloudy thinking or misunderstanding, then it is likely others who are interested in in the topic will benefit from our attempt to clarify ideas or resolve problems.