Writing helps you clarify your thinking in terms of relevance, reasoning, and accuracy.
At high school, most learning occurs at the levels of Knowledge, Understanding and Application.
For example, you may be expected to learn the names and properties of chemical elements (knowledge), understand why some react with others (understanding) and conduct experiments (application).
At these levels, memory and comprehension are necessary, and remain so at university. Markers often write comments on assignments which only communicate these lower levels of thinking, such as; "this is just description", "analyse this in more detail", or "you haven't understood the issues".
Bloom's Taxonomy of Intellectual Behaviour - or the thinking you need to do at university*The pyramid below shows the levels of thinking skills expected at university in all disciplines of study.