Essays / Is it better if I gear my essay explicitly towards why I wanted to study engineering? [8]

Hey! First time using essay forum and was wondering if anyone could tell me whether it would be better if the common application essay we write is about what major we intend to study or does it not matter?

For example, if I want to study engineering, is it better if I gear my essay towards why I wanted to study engineering, or can it just be about anything? Thank you very much in advance! =)