Observation vs construction
Often when I see students discussing different approaches to drawing, observational drawing and constructional drawing are presented as a sort of dichotomy - two techniques that are mutually exclusive. This really isn't true. Constructional drawing inherently incorporates observation, as we cannot know which simple forms to start with unless we observe our subject matter carefully. Observational drawing can be done without construction, but if you ask me, that is an approach that is fundamentally incorrect.
Construction is all about understanding how an object exists in 3D space as a part of the drawing process. If you are doing this - even if not as explicitly as we do here - you are employing an element of construction.
If, however, you are working only in two dimensions - for example, drawing from a photo reference (photographs are by nature two dimensional) and reproducing it in your drawing directly without considering the fact that the photo represents a three dimensional scene, then there is no component of construction and in all likelihood your drawing will appear flat and unconvincing. Yes, you may eventually become an exceptional photocopier, but we do have machines for that already, and the applications for that skill set are fairly limited these days.
This is why if you have learned any observational drawing in the past, you may have heard discussion over whether or not it is okay to use photo references, or if only drawing "from life" (with the actual object in front of you) is valuable. In my experience, this is because it is considerably more difficult for a beginner to look at an actual three dimensional object without some degree of understanding or consideration for how it sits in 3D space. You're effectively forced to somehow process that 3D object into your 2D drawing.
I have generally found that when employing constructional techniques, like those taught here, it doesn't matter as much whether you draw them from life or not. There are still benefits to drawing an object from life, and if you have the means or the opportunity you absolutely should do so, but these techniques do help significantly reduce the pitfalls that come from working with 2D references.