I'm just curious why none of the Templates allows us to use all the space available on the monitor/screen?
Is there an something somewhere that indicates what rules/guidelines must be followed if a WET template is not used?
Full screen width has a readability problem with text, as recently discussed in https://github.com/wet-boew/GCWeb/issues/1239.
I'm referring to using all the space for navigation AND content though...
There a huge difference between a page like this which is about 50% white space and the screen capture attached

Full width template would be great for full featured web applications. The current templates are optimized mostly for content driven websites.
I agree, it would be very useful.
3 years later, I see a new need for this. 7 years before, somebody else did in https://github.com/wet-boew/wet-boew/issues/3025.
IMO it's kind of strange that WET/GCWeb don't have any "official" full width page templates...
But in practice it's pretty straightforward to switch to full width. Just need to rename the container class to container-fluid. That class was introduced in Bootstrap 3.1.0 and has been supported by WET 4.0.x since #4549 (February 2014). It can even be used in the content areas of Canada.ca (MWS) pages by ticking a "Use fluid width" option in page properties.
WET's CSS also contains a media query for extra-large view (1600px and up)... but it's very barebones.
Here's its main SCSS code:
https://github.com/wet-boew/wet-boew/blob/ae69c1027e1661a6377a29f52e58c3cc1f3476d8/src/base/_wet-boew.scss#L240-L244
So it's practically non-existent at this time. It's not in a state where you could expect to use it in the same manner as Bootstrap 3.x's regular view classes (e.g. col-xl-6).
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I'm referring to using all the space for navigation AND content though...
There a huge difference between a page like this which is about 50% white space and the screen capture attached