First of all: that plugin just needs a single click to activate it. Then you can forget it. That saves time.
Second: the plugin provides more informations in the viewport meta tag than your code. Enabled zooming is more probable. Compare both:
your code
<meta name="viewport" content="width=device-width, initial-scale=1, maximum-scale=1" />
vs. the plugin’s code
<meta name="viewport" content="width=device-width, user-scalable=yes, initial-scale=1.0, minimum-scale=0.1, maximum-scale=10.0">
You can find useful informations about that tag here.
Third: if you switch to another theme, then your code is no more printed out. The plugin on the other side is still in effect after every theme switch.