Why does one need to usermod to www-data

In many tutorials around the DO knowledge base you get a suggestion to first do

sudo usermod -aG www-data $USER

and then do

sudo chown -R www-data:www-data /var/www/

In one of my older droplets all I did was simply

sudo chown -R $USER /var/www/

and never had any problems.

Am I missing something here?