Extra edge over peers - Students preparing for JEE since class 10 will definitely have extra benefits over their friends.
No need of joining special coaching classes - A lot of students who study for JEE from class 10 generally join special coaching classes in order to cover the syllabus fast.
Be a confident student - By preparing early, the confidence level of the students increases.
Sufficient time - The ones who started preparation early will have plenty of time in hand to work on the finer aspects of difficult questions.