If global variable should be avoided in most cases, why does Dave continue to define global variables rather than local?

Chyno is correct. If specifically you mean when he does something like this?

var student;

function getStudentReport( student ){
    var report = '<h2>Student: ' + student.name + '</h2>';
    report += '... more student message stuff...';
    return report;
}

If so, the student parameter used in the function is not the same as the global variable student. With parameters, the function takes whatever is passed to it, like this

function print( message ){
    console.log(message);
}

print('hello world');

and quietly in the background inside the function treats it like a local variable like this

{
    //inside the function called via print('hello world');
    var message = 'hello world';
    console.log(message);
}

If you are referring to the use of the students variable inside the while loop: while loops don't have their own scope. Only the functions have scope. Any variables declared inside the loop would still be global. Plus it would be less efficient as the variable would keep being declared in every loop iteration. Thats why search and message are declared at the top and not in the while and for loops.

For now, I think that he is stressing the idea of "not using global variables within functions" more than "not using global variables at all". There will be more advanced techniques to avoid polluting the global scope down the line like immediately invoked function expressions (IFFE) but that might be unnecessarily confusing in a beginner level course.

For example:

;(function(){

    /*
    now student is not a global variable, but dave has
    to go deeper into scope and other things that are not
    necessary at this point */
    var student;

    function getStudentReport( student ){
        var report = '<h2>Student: ' + student.name + '</h2>';
        report += '... more student message stuff...';
        return report;
    }

})();

While it is true that you should avoid global variables, In some cases, you'll need to have access to specific variables within different areas in your scripts.

If a variable is only used in one function and does not need to be access elsewhere. then the best thing would be to make it a local variable to the specific function.

Thank you! I definitely missed this: "while loops don't have their own scope. Only the functions have scope."

So loops do not have scope? That's extremely helpful. I just assumed that because it fell within the {}, local scope would apply.