Highlights, by themselves, are just fleeting notes that do not elaborate on a text and do not extend your knowledge. When they accumulate, they become useless unless they are processed. The same can be said about marginalia and other annotations.
Instead of just highlighting while reading, aspire to write permanent notes about what you read to test understanding. Since it can be difficult to go back and forth between reading and writing, you can use highlighting to mark parts of a text that could be converted into permanent notes on a second reading as is done in the barbell method of reading.
If indeed, you make a highlight or take any form of fleeting notes, put them in inboxes to capture fleeting notes and have a mechanism in place to drain them regularly. For example, elaborate fleeting notes within a day or two or use a spaced repetition writing system to drain a writing inbox.
References
Ahrens, S. (2017). How to Take Smart Notes: One Simple Technique to Boost Writing, Learning and Thinking – for Students, Academics and Nonfiction Book Writers. CreateSpace Independent Publishing Platform.