Is there any specific reason behind using $ with variable in jQuery

No there is no real difference.

It’s just a convention that helps you remember that a isn’t the DOM element but it’s a jQuery object.

var a = document.getElementById('a');
a.innerHTML  //fine

var $a = $('#a');
$a.html()   // fine

Ohhh, and by the way, neither a or $a are good variable names … you should use meaningful variable names not abc characters.

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Read the jQuery info tag on this very same site:

Variable Naming Conventions

jQuery wrapped variables are usually named starting with ‘$’ to distinguish them from standard JavaScript objects.

var $this = $(this);