Some subjects require them.
Some universities use them for filtering.
Some admissions staff read them.
Sometimes they're used for interviews.
Sometimes they're used to differentiate in confirmation.
The exceptions make them a compulsory part of the application for everyone.
FWIW it's a useful exercise even if they aren't read. If an applicant struggles to express why they want to study a subject then committing to 3+ years isn't likely a good plan....and when it comes to job applications learning to write about your interests, activities and abilities in this way is important.