Highlighting certain parts in textbooks helps the person remember the important parts later.
Using highlighters in textbooks is considered fun by some people as they find it an enjoyable activity to color words or lines.
Some people like making their textbooks colorful to make them feel better about studying.
Some people use different colors to highlight their studying materials so they remember them by color in an exam.
Some people underline whatever they're studying as a way of concentrating, while other people use highlighters.
Some people use highlighters to make it easier for themselves to revise the material later on by focusing only on the important parts.
Some people underline and highlight texts while studying to make themselves feel like they accomplished something since the material looks like it's been studied.