Is there is any advantage of using the get_query_var() over $_GET()?

I have tried like this

myurl?posts_per_page=2

and the results are totally different

echo $_GET['posts_per_page'] // output is 2
echo get_query_var('posts_per_page') //output is 10

I have not added ‘posts_per_page’ to query var list using action so where does this 10 is coming up from?